This takes advantage of CSS variables and color expressions to improve
the `window-theme=ghostty` support. The only visibile difference from
the previous implementation is that the header bar will darken if the
Ghostty window is in the background, which is standard for GTK apps.
This is conditional at runtime. If Ghostty detects that you're running
against GTK 4.16 or newer it will use the CSS variables and color calcs.
If you're running against older versions it will use CSS classes to
achieve nearly the same effect.
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