Add more color aliases that take dark mode into account

We should add a few new aliases for our colors, e.g., --purple-60 and --purple-30, to remove more media queries.

They're needed for example in the pereferences (see !660 (comment 2909188)).

We already do it for --tor-branding-color, but it's tied to the release channel.

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mentioned in merge request !660 (merged)

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I think I'll defer to @henry's judgement here as to whether that's a good idea, or if explicit styles for prefers dark and high contrast mode is easier to maintain.

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mentioned in issue #42752 (closed)

mentioned in merge request !1109 (merged)

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