diff --git a/CHANGELOG.adoc b/CHANGELOG.adoc
index 8cda4fcee..55781c0f3 100644
--- a/CHANGELOG.adoc
+++ b/CHANGELOG.adoc
@@ -7,7 +7,9 @@ For a detailed view of what has changed, refer to the {url-repo}/commits/main[co
 
 == Unreleased
 
-_No changes since previous release._
+Improvements::
+
+* replace OpenStruct with internal ThemeData class for storing theme data (#2535)
 
 == 2.3.18 (2024-07-27) - @mojavelinux
 
diff --git a/Gemfile b/Gemfile
index e46a41e10..fc22f4e55 100644
--- a/Gemfile
+++ b/Gemfile
@@ -9,6 +9,7 @@ gem 'asciidoctor', ENV['ASCIIDOCTOR_VERSION'], require: false if ENV.key? 'ASCII
 gem 'asciidoctor-diagram', ENV['ASCIIDOCTOR_DIAGRAM_VERSION'], require: false if ENV.key? 'ASCIIDOCTOR_DIAGRAM_VERSION'
 gem 'asciidoctor-kroki', ENV['ASCIIDOCTOR_KROKI_VERSION'], require: false if ENV.key? 'ASCIIDOCTOR_KROKI_VERSION'
 gem 'coderay', '~> 1.1.0', require: false
+gem 'logger', require: false if (Gem::Version.new RUBY_VERSION) > (Gem::Version.new '3.3.4')
 gem 'open-uri-cached', '~> 1.0.0', require: false
 gem 'prawn-gmagick', ENV['PRAWN_GMAGICK_VERSION'], require: false if (ENV.key? 'PRAWN_GMAGICK_VERSION') && RUBY_ENGINE == 'ruby'
 gem 'pygments.rb', ENV['PYGMENTS_VERSION'], require: false if ENV.key? 'PYGMENTS_VERSION'
@@ -35,5 +36,6 @@ end
 
 group :coverage do
   gem 'deep-cover-core', '~> 1.1.0', require: false
+  gem 'json', '~> 2.7', require: false if (Gem::Version.new RUBY_VERSION) > (Gem::Version.new '3.3.4')
   gem 'simplecov', '~> 0.22.0', require: false
 end
diff --git a/lib/asciidoctor/pdf/theme_data.rb b/lib/asciidoctor/pdf/theme_data.rb
new file mode 100644
index 000000000..b487cfc6d
--- /dev/null
+++ b/lib/asciidoctor/pdf/theme_data.rb
@@ -0,0 +1,57 @@
+# frozen_string_literal: true
+
+module Asciidoctor
+  module PDF
+    class ThemeData
+      attr_reader :table
+
+      def initialize data = nil
+        @table = (data || {}).transform_keys(&:to_sym)
+      end
+
+      def [] name
+        @table[name.to_sym]
+      end
+
+      def []= name, value
+        @table[name.to_sym] = value
+      end
+
+      def each_pair &block
+        @table.each_pair(&block)
+      end
+
+      def eql? other
+        @table.to_h.eql? other.to_h
+      end
+
+      def delete_field name
+        @table.delete name
+      end
+
+      def dup
+        ThemeData.new @table
+      end
+
+      def method_missing name, *args
+        if (name_str = name.to_s).end_with? '='
+          @table[name_str.chop.to_sym] = args[0]
+        else
+          @table[name]
+        end
+      end
+
+      def respond_to? name, _include_all = false
+        @table.key? name.to_sym
+      end
+
+      def respond_to_missing? name, _include_all = false
+        @table.key? name.to_sym
+      end
+
+      def to_h
+        @table
+      end
+    end
+  end
+end
diff --git a/lib/asciidoctor/pdf/theme_loader.rb b/lib/asciidoctor/pdf/theme_loader.rb
index 12dffc8d2..30613aa06 100644
--- a/lib/asciidoctor/pdf/theme_loader.rb
+++ b/lib/asciidoctor/pdf/theme_loader.rb
@@ -1,6 +1,6 @@
 # frozen_string_literal: true
 
-require 'ostruct'
+require_relative 'theme_data'
 require_relative 'measurements'
 
 module Asciidoctor
@@ -69,7 +69,7 @@ def self.resolve_theme_asset asset_path, theme_dir = nil
       # NOTE: base theme is loaded "as is" (no post-processing)
       def self.load_base_theme
         ::File.open BaseThemePath, mode: 'r:UTF-8' do |io|
-          (::OpenStruct.new ::YAML.safe_load io, filename: BaseThemePath).tap {|theme| theme.__dir__ = ThemesDir }
+          (ThemeData.new ::YAML.safe_load io, filename: BaseThemePath).tap {|theme| theme.__dir__ = ThemesDir }
         end
       end
 
@@ -78,7 +78,7 @@ def self.load_theme theme_name = nil, theme_dir = nil
         if theme_path == BaseThemePath
           load_base_theme
         else
-          theme_data = load_file theme_path, (::OpenStruct.new base_font_size: 12), theme_dir
+          theme_data = load_file theme_path, (ThemeData.new base_font_size: 12), theme_dir
           unless (::File.dirname theme_path) == ThemesDir
             theme_data.base_text_align ||= 'left'
             theme_data.base_line_height ||= 1
@@ -102,12 +102,12 @@ def self.load_file filename, theme_data = nil, theme_dir = nil
           line.sub(HexColorEntryRx) { %(#{(m = $~)[:k]}: #{m[:h] || (m[:k].end_with? 'color') ? "'#{m[:v]}'" : m[:v]}) }
         end.join unless (::File.dirname filename) == ThemesDir
         yaml_data = ::YAML.safe_load data, aliases: true, filename: filename
-        (loaded = (theme_data ||= ::OpenStruct.new).__loaded__ ||= ::Set.new).add filename
+        (loaded = (theme_data ||= ThemeData.new).__loaded__ ||= ::Set.new).add filename
         if ::Hash === yaml_data && (extends = yaml_data.delete 'extends')
           (Array extends).each do |extend_path|
             extend_path = extend_path.slice 0, extend_path.length - 11 if (force = extend_path.end_with? ' !important')
             if extend_path == 'base'
-              theme_data = ::OpenStruct.new theme_data.to_h.merge load_base_theme.to_h if (loaded.add? 'base') || force
+              theme_data = ThemeData.new theme_data.to_h.merge load_base_theme.to_h if (loaded.add? 'base') || force
               next
             elsif BundledThemeNames.include? extend_path
               extend_path, extend_theme_dir = resolve_theme_file extend_path, ThemesDir
@@ -123,7 +123,7 @@ def self.load_file filename, theme_data = nil, theme_dir = nil
       end
 
       def load hash, theme_data = nil
-        ::Hash === hash ? hash.reduce(theme_data || ::OpenStruct.new) {|data, (key, val)| process_entry key, val, data, true } : (theme_data || ::OpenStruct.new)
+        ::Hash === hash ? hash.reduce(theme_data || ThemeData.new) {|data, (key, val)| process_entry key, val, data, true } : (theme_data || ThemeData.new)
       end
 
       private
diff --git a/spec/theme_loader_spec.rb b/spec/theme_loader_spec.rb
index 2916bd938..68b8b21b7 100644
--- a/spec/theme_loader_spec.rb
+++ b/spec/theme_loader_spec.rb
@@ -9,26 +9,26 @@
     it 'should not fail if theme data is empty' do
       theme = subject.new.load ''
       (expect theme).not_to be_nil
-      (expect theme).to be_an OpenStruct
+      (expect theme).to be_an Asciidoctor::PDF::ThemeData
       (expect theme.to_h).to be_empty
     end
 
     it 'should not fail if theme data is falsy' do
       theme = subject.new.load false
       (expect theme).not_to be_nil
-      (expect theme).to be_an OpenStruct
+      (expect theme).to be_an Asciidoctor::PDF::ThemeData
       (expect theme.to_h).to be_empty
     end
 
     # NOTE: this API is not used by the converter
     it 'should use specified theme data if raw theme data is nil' do
-      theme_data = OpenStruct.new
+      theme_data = Asciidoctor::PDF::ThemeData.new
       theme_data.base_font_color = '222222'
       theme = subject.new.load nil, theme_data
       (expect theme).to be theme_data
     end
 
-    it 'should store flattened keys in OpenStruct' do
+    it 'should store flattened keys in Asciidoctor::PDF::ThemeData' do
       theme_data = YAML.safe_load <<~'EOS'
       page:
         size: A4
@@ -41,7 +41,7 @@
           font_style: bold
       EOS
       theme = subject.new.load theme_data
-      (expect theme).to be_an OpenStruct
+      (expect theme).to be_an Asciidoctor::PDF::ThemeData
       (expect theme).to respond_to :page_size
       (expect theme).to respond_to :base_font_family
       (expect theme).to respond_to :base_border_width
@@ -62,7 +62,7 @@
             size: 24
       EOS
       theme = subject.new.load theme_data
-      (expect theme).to be_an OpenStruct
+      (expect theme).to be_an Asciidoctor::PDF::ThemeData
       (expect theme.admonition_icon_tip).to be_a Hash
       (expect theme.admonition_icon_tip).to eql name: 'far-lightbulb', stroke_color: 'FFFF00', size: 24
       (expect theme.admonition_icon_note).to be_a Hash
@@ -76,7 +76,7 @@
           advice: ~
       EOS
       theme = subject.new.load theme_data
-      (expect theme).to be_an OpenStruct
+      (expect theme).to be_an Asciidoctor::PDF::ThemeData
       (expect theme.admonition_icon_advice).to be_nil
     end
 
@@ -93,7 +93,7 @@
             stroke-color: FFFF00
       EOS
       theme = subject.new.load theme_data
-      (expect theme).to be_an OpenStruct
+      (expect theme).to be_an Asciidoctor::PDF::ThemeData
       (expect theme).to respond_to :page_size
       (expect theme).to respond_to :base_font_family
       (expect theme).to respond_to :abstract_title_font_size
@@ -112,7 +112,7 @@
             color: 0000ff
       EOS
       theme = subject.new.load theme_data
-      (expect theme).to be_an OpenStruct
+      (expect theme).to be_an Asciidoctor::PDF::ThemeData
       (expect theme).to respond_to 'role_flaming-red_font_color'
       (expect theme['role_flaming-red_font_color']).to eql 'FF0000'
       (expect theme).to respond_to 'role_so-very-blue_font_color'
@@ -126,7 +126,7 @@
           font-style: bold
       EOS
       theme = subject.new.load theme_data
-      (expect theme).to be_an OpenStruct
+      (expect theme).to be_an Asciidoctor::PDF::ThemeData
       (expect theme).to respond_to 'role_BOLD_font_style'
       (expect theme['role_BOLD_font_style']).to eql 'bold'
     end
@@ -148,7 +148,7 @@
             content: 2 * 2
       EOS
       theme = subject.new.load theme_data
-      (expect theme).to be_an OpenStruct
+      (expect theme).to be_an Asciidoctor::PDF::ThemeData
       (expect theme.menu_caret_content).to eql '[">"]'
       (expect theme.ulist_marker_disc_content).to eql '0'
       (expect theme.footer_recto_left_content).to eql 'true'
@@ -172,7 +172,7 @@
           text-align: $heading-align
         EOS
         theme = subject.new.load theme_data
-        (expect theme).to be_an OpenStruct
+        (expect theme).to be_an Asciidoctor::PDF::ThemeData
         (expect theme.base_align).to be_nil
         (expect theme.base_text_align).to eql 'center'
         (expect theme.heading_align).to be_nil
@@ -197,7 +197,7 @@
             end: $table-caption-side
         EOS
         theme = subject.new.load theme_data
-        (expect theme).to be_an OpenStruct
+        (expect theme).to be_an Asciidoctor::PDF::ThemeData
         (expect theme.table_caption_side).to be_nil
         (expect theme.table_caption_end).to eql 'bottom'
         (expect theme.image_caption_end).to eql 'bottom'
@@ -214,7 +214,7 @@
           item-spacing: $outline_list_item_spacing / 2
         EOS
         theme = subject.new.load theme_data
-        (expect theme).to be_an OpenStruct
+        (expect theme).to be_an Asciidoctor::PDF::ThemeData
         (expect theme.outline_list_item_spacing).to be_nil
         (expect theme.list_item_spacing).to eql 6
         (expect theme.footnotes_margin_top).to eql theme.list_item_spacing
@@ -232,7 +232,7 @@
           font-color: $blockquote-font-color
         EOS
         theme = subject.new.load theme_data
-        (expect theme).to be_an OpenStruct
+        (expect theme).to be_an Asciidoctor::PDF::ThemeData
         (expect theme.blockquote_font_color).to be_nil
         (expect theme.quote_font_color).to eql '4A4A4A'
         (expect theme.quote_border_color).to eql theme.quote_font_color
@@ -249,7 +249,7 @@
           font-color: $key-border-color
         EOS
         theme = subject.new.load theme_data
-        (expect theme).to be_an OpenStruct
+        (expect theme).to be_an Asciidoctor::PDF::ThemeData
         (expect theme.key_border_color).to be_nil
         (expect theme.kbd_border_color).to eql 'CCCCCC'
         (expect theme.kbd_font_color).to eql theme.kbd_border_color
@@ -265,7 +265,7 @@
           font-family: $literal-font-family
         EOS
         theme = subject.new.load theme_data
-        (expect theme).to be_an OpenStruct
+        (expect theme).to be_an Asciidoctor::PDF::ThemeData
         (expect theme.literal_font_family).to be_nil
         (expect theme.codespan_font_family).to eql 'M+ 1mn'
         (expect theme.verse_font_family).to eql 'M+ 1mn'
@@ -284,7 +284,7 @@
         padding: [6, 12, -6, 14]
       EOS
       theme = subject.new.load theme_data
-      (expect theme).to be_an OpenStruct
+      (expect theme).to be_an Asciidoctor::PDF::ThemeData
       (expect theme.example_padding).to eql [12, 12, 12, 12]
       (expect theme.quote_padding).to eql [0, 12, 0, 14]
       (expect theme.sidebar_padding).to eql [12, 12, 12, 12]
@@ -297,7 +297,7 @@
         padding: [-3, 12, -3, 14]
       EOS
       theme = subject.new.load theme_data
-      (expect theme).to be_an OpenStruct
+      (expect theme).to be_an Asciidoctor::PDF::ThemeData
       (expect theme.quote_padding).to eql [-3, 12, -3, 14]
     end
 
@@ -315,7 +315,7 @@
             content: $page_size
       EOS
       theme = subject.new.load theme_data
-      (expect theme).to be_an OpenStruct
+      (expect theme).to be_an Asciidoctor::PDF::ThemeData
       (expect theme.footer_verso_left_content).to eql '2 * 12'
       (expect theme.footer_verso_right_content).to eql 'A4'
     end
@@ -477,16 +477,16 @@
   describe '.load_file' do
     it 'should not fail if theme file is empty' do
       theme = subject.load_file fixture_file 'empty-theme.yml'
-      (expect theme).to be_an OpenStruct
+      (expect theme).to be_an Asciidoctor::PDF::ThemeData
       theme.delete_field :__loaded__
-      (expect theme).to eql OpenStruct.new
+      (expect theme).to eql Asciidoctor::PDF::ThemeData.new
     end
 
     it 'should not fail if theme file resolves to nil' do
       theme = subject.load_file fixture_file 'nil-theme.yml'
-      (expect theme).to be_an OpenStruct
+      (expect theme).to be_an Asciidoctor::PDF::ThemeData
       theme.delete_field :__loaded__
-      (expect theme).to eql OpenStruct.new
+      (expect theme).to eql Asciidoctor::PDF::ThemeData.new
     end
 
     it 'should throw error that includes filename and reason if theme is indented using tabs' do
@@ -726,7 +726,7 @@
     it 'should load base theme if theme name is base' do
       theme = subject.load_theme 'base'
       (expect theme).not_to be_nil
-      (expect theme).to be_an OpenStruct
+      (expect theme).to be_an Asciidoctor::PDF::ThemeData
       (expect theme.base_font_family).to eql 'Helvetica'
       (expect theme.codespan_font_family).to eql 'Courier'
       (expect theme).to eql subject.load_base_theme
@@ -735,7 +735,7 @@
     it 'should load default theme if no arguments are given' do
       theme = subject.load_theme
       (expect theme).not_to be_nil
-      (expect theme).to be_an OpenStruct
+      (expect theme).to be_an Asciidoctor::PDF::ThemeData
       (expect theme.base_font_family).to eql 'Noto Serif'
       (expect theme.link_font_color).to eql '428BCA'
     end