BlueOS: Theme: document user css location

[ES-Alexander]

• Relevant PR: create "userdata" folder for allowing customizations BlueOS#1419
• Should be linked to from extensions docs on theming, but is technically a user feature (similar to BlueOS: Theme: document 3D model override process #66)
• Custom stylesheet location: userdata/styles/theme_style.css
  • can access/modify via the File Browser
  • may need to create folders/files if they don't already exist
  • the colors update at the next page refresh after the file is saved
• Structure:

  :root {
      --v-primary-base: #c0ffee !important;
      --v-info-base: forestgreen !important;
  }

  • specified using css colors
    • colorhexa may be useful for palette creation / accessibility checking
  • the !important tag is required for a color to properly override its default
• Most relevant colors to change for a re-skin:
  • --v-primary-base
    • controls header bar, sidebar highlights, and submit button colors
    • best to set a color with lightness ~50%, for good compatibility with both light and dark viewing modes
  • --v-info-base
    • info boxes
    • generally set to the same as --v-primary-base
  • --v-error-base
    • notification bell and pirate mode icon colors
• Others that might be of interest:
  • --v-background-base
  • --v-surface-base
  • --v-secondary-base
  • --v-success-base
  • --v-warning-base