stylesheet: Brighten accent color for dark variant

We finally can do it.

Remove a bunch of dark variant overrides as well.

Fixes #182
4 jobs for wip/exalm/buttons-5 in 2 minutes and 13 seconds (queued for 3 seconds)
Status Job ID Name Coverage
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passed #1342073
abi-check

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passed #1342070
api-visibility

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passed #1342072
flatpak
build-flatpak

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passed #1342071
flatpak
doc

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