UI/themes: Fix selectors in Yami Light

[gxalpha]

Description

Fixes a few QSS selectors in Yami Light.

Amends cb02696 (#11226).

Motivation and Context

Found when investigating an issue raised in the beta-testing on Discord.

How Has This Been Tested?

Before/After:

git grep'ed for =true] to make sure this type selector wasn't used anywhere else.

Types of changes

• Bug fix (non-breaking change which fixes an issue)

Checklist:

• My code has been run through clang-format.
• I have read the contributing document.
• My code is not on the master branch.
• The code has been tested.
• All commit messages are properly formatted and commits squashed where appropriate.
• I have included updates to all appropriate documentation.