syntax highlighting of code blocks only works for default theme

[nylki]

Describe the bug
Code blocks (monospace blocks with syntax highlighting) don't work for most themes, only the default theme.

To Reproduce
Steps to reproduce the behavior:

1. open an ePub that has a code block with syntax highlighting
2. Click on "Fonts and Layout" (settings top right)
3. click on "Colors" (tab bar item)
4. Select a different color theme (other than default)
5. code block is not syntax highlighted anymore, using the same font color as the regular text

Expected behavior
When changing the color theme I expect syntax highlight of code blocks to still be visible.

Screenshots
default theme, featuring syntax highlighting and different bg-color:

solarized (or sepia) theme without syntax highlighting and same bg-color:

Version:

• Foliate version:
• OS/Distribution and version: Fedora Linux 40 (Workstation Edition)
• Desktop environment: Gnome 46
• Installation method: Official Fedora repo RPM (but also checked on the flathub version)

[johnfactotum]

This is a feature to ensure that the text is legible. I think this is also the behavior in Apple Books and Readium/Thorium.

I suppose the easiest way out would be to just add an option to disable this.

[nylki]

This is a feature to ensure that the text is legible. I think this is also the behavior in Apple Books and Readium/Thorium.

I suppose the easiest way out would be to just add an option to disable this.

Thank you for your quick response! Just to avoid confusion: Is my assumption correct that the non-default themes don't have a code-block specific stylesheet that would fit their color scheme? (so they are colored as if they are plaintext).

[johnfactotum]

The syntax highlighting is part of the stylesheet of the EPUB file. Most EPUB files are designed to be displayed on a white background only and look broken when you apply a custom theme. So when using non-white background, Foliate overrides all foreground and background colors so that the theme would be applied consistently and the text would remain readable. See 29eed7e.

[nylki]

@johnfactotum Thanks for explaining, makes way more sense now 🙂 ! I did not know that the syntax coloring is baked into the EPUB itself.
I was hoping that code blocks were somehow defined in a more generic way in EPUB (like a <code></code>-block), so that applying a syntax theme would be relatively easy.

I wonder if it would be feasible to heuristically identify code blocks and then define a custom theme for those section.

Anyways, feel free to close the issue if you like.

[johnfactotum]

Yes, the code should be marked up with the <code> element. However, to quote the HTML Standard:

There is no formal way to indicate the language of computer code being marked up.

So automatically applying syntax highlighting would be problematic.

[nylki]

I see. So, just to entertain the idea: although there is no formal way, auto-detection of the language inside a block could be an option (perhaps as an option that is disabled by default, or more involved, via a button to change the detected language). For example highlight.js does supports auto-detection of languages.

[johnfactotum]

Yes, that would be possible in theory, but probably too localized and complicated to be ever implemented in Foliate itself.

Some alternatives, when the book already has syntax highlighting, which most of them do, are

• Allow disabling color overrides
• Use other methods such as filters to change the colors
• Bring back support for __ibooks_internal_theme, and then the book itself can use -webkit-text-fill-color to force the text color