Tweaks: Improve contributed content tweak compatibility

[marcustyphoon]

Description

As noted in the linked issue, the "highlight contributed content" tweak looks like it applies a subtle color highlight to the relevant trail item, but it's actually overriding the background with the follow palette color, which generally has that effect on most palettes but isn't compatible with customization.

This applies a low-opacity accent color instead. 7% matches more closely with the current color on the True Blue palette, but I like ~10% better in general.

I also tried blue instead of accent.

I prefer blue on:
cement, strongly

I prefer deprecated-accent on:
vampire, moderately
goth rave, slightly

Most notably, though we override accent in Themed Posts, but we don't override blue, so accent gives more stable results.

In addition, this disables the tweak's effect entirely when Themed Posts' "Theme every reblog trail item individually" setting is enabled; I guess it could make sense not to do this since it'll still work when people have default blog themes? It seems like a lot of the time it would be pointless, though.

Resolves #873.

Testing steps

The "alternative color demo" commit demos a way to add a second color option and compare side-by-side; one can find a post with a decent reblog chain and shift-P through different palettes.

• Confirm that contributed content trail items look reasonable with Themed Posts disabled in each palette.
• Confirm that contributed content trail items look reasonable with Themed Posts enabled. (Changing palettes should have no effect.)