Migrate CSS highlight to CSS variables

[GuillaumeGomez]

Part of #98460.

There are UI changes: I unified the colors as follows:

• All keyword elements now have the same color.
• All the literal elements now have the same color.
• I removed the italic on .self in the ayu theme as it was the only theme with it.

You can test it here.

cc @jsha
r? @notriddle

[rustbot]

Some changes occurred in HTML/CSS/JS.

cc @GuillaumeGomez, @Folyd, @jsha

A change occurred in the Ayu theme.

cc @Cldfire

[notriddle]

I don't think the color changes are an improvement:

• Prelude items are not keywords. You can shadow them, they're part of the regular grammar, and many of them are plain library code with no special lang-item powers. Implying that None and fn are in some way the same thing seems like it would confuse beginners.

• Is making all keywords the same color actually a positive? It seems like the old color scheme intentionally made &, *, and mut their own color, separate from all other "keywords", because they're so heavily used. Making them the same color means, in function signatures, half the code is getting highlighted with the same shade of purple, which mostly defeats the purpose of syntax highlighting.

  

• If I was going to tweak the way literal expressions are highlighted, I'd make bool literals and numeric literals the same color, while giving string literals a separate color. Strings are special, because they can be arbitrarily long and contain text that looks like syntax ("false" and false are both valid), so it makes sense to want to distinguish them.

[GuillaumeGomez]

I'll update following your suggestions then to see if it looks better.

[GuillaumeGomez]

I updated the style as you recommended. I tried to pick good colours for literals but don't hesitate if you have better ones. :)

[bors]

☔ The latest upstream changes (presumably #102741) made this pull request unmergeable. Please resolve the merge conflicts.

[GuillaumeGomez]

Fixed merge conflict.

[notriddle]

Opened a zulip thread for this @ https://rust-lang.zulipchat.com/#narrow/stream/266220-rustdoc/topic/Syntax.20highlighting.20.23102663

[jsha]

In general we should not combine refactorings with nontrivial feature changes. Can you make the refactoring (change to CSS variables) without making the feature change (merging colors)? Then we can discuss the feature change independently.

[GuillaumeGomez]

It's a bit complicated because ayu combines colors differently than the other themes, hence why I tried to clarify things a bit. I'll try to keep the current behaviour as much as possible then.