Fix accent colors not applying

[marcustyphoon]

User-facing changes

Fixes accent colors not being affected by the extension due to a Tumblr code change that I didn't really look into.

Technical explanation

Applies what used to be an accent override to the deprecated-accent CSS variable as well; this results in no changes being needed to custom palettes as well as built-in palette data.

I assume the change to this variable name could be a sign of more things to come that could be problematic for custom palettes still working without tweaks/migration? But if so, that can be a problem for later.

Issues this closes

Resolves #183, resolves #182

[marcustyphoon]

Ah, I see. It looks like --accent is being repurposed into a part of a system of colors with variations, and that in that system the variables are colors rather than numerical triplets?

That I can tell, these new colors appear to be hardcoded; if that's the case I assume we don't want to touch them, as overriding only some would lead to inconsistency. Thus, if my interpretation is right we should be sure to only modify --deprecated-accent, and decline to modify --accent? I should make a second tweak to new-xkit/XKit#2141 if so.