Fix build errors due to invalid CSS

[HolgerKnublauch]

See https://github.com/w3c/data-shapes/actions/runs/18511032788/job/52751744800

    "file:/home/runner/work/data-shapes/data-shapes.w3c/shacl12-core/index.html":6165.5-6165.72: error: CSS: “background”: “light-dark(#f3f3f3,#2b2b2b)” is not a “background-color” value.
    "file:/home/runner/work/data-shapes/data-shapes.w3c/shacl12-core/index.html":6166.6-6166.72: error: CSS: “color”: “#0000a0” is not a “color” value.
    "file:/home/runner/work/data-shapes/data-shapes.w3c/shacl12-core/index.html":6177.6-6177.90: error: CSS: “color”: “#0000a0” is not a “color” value.

[HolgerKnublauch]

Hi @smessie is this something you know about? It seems to have started with your commit 3 weeks ago. ChatGPT indicates something about "The light-dark() color function is part of an upcoming CSS Color Module Level 5 feature, but: It’s not yet widely supported in most browsers (as of 2025, only experimental in Safari and behind flags in Chrome/Firefox)."

I see the same styles are also defined in the common/style.css so maybe those definitions can be reused instead?

[smessie]

According to the Mozilla docs, the feature light-dark() is already widely adopted since 2024, so I would guess it is an issue with the validator being used.
https://developer.mozilla.org/en-US/docs/Web/CSS/color_value/light-dark

[HolgerKnublauch]

Whatever policy the validator applies unfortunately breaks the release process and the drafts haven't been published since early September.

[caribouW3]

It will be implemented in the CSS Validator, but in the meantime it's best not to use it.