Add color-scheme: dark to fix #2554: browser not detecting dark mode

[SodaWithoutSparkles]

What does this PR aim to accomplish?:

fix #2554: browser not detecting that dark mode is already used and attempts to apply force-dark-mode, ruining the site's look

How does this PR accomplish the above?:

Add color-scheme: dark; to the html section of all .css files related to dark mode.

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[SodaWithoutSparkles]

Shall I delete the branch now that it is merged?

[yubiuser]

The branch is on your fork, you can do as you want.