Popup: Highlight preferences on search

[marcustyphoon]

Description

Implements a super-simple highlight around preference options that match the current search.

Maybe this should be a different color in dark mode? Probably. I'm not much of a dark mode user, so maybe someone could weigh in on what feels appropriate. (Honestly, I'm pretty meh on the light mode color too.)

Resolves #1111

Testing steps

[marcustyphoon]

(font-weight: bold is another option.)

[AprilSylph]

Hmm, maybe the yellow highlight is too much on dark mode. Firefox highlights matching text when searching for settings with that yellow on light mode, but with #003eaa (RGB: 0, 62, 170) on dark mode.

[marcustyphoon]

Ah, now I remember why I had added that line to the dark mode section :D

[marcustyphoon]

An option:

.script li.search-highlighted {
  background: rgb(var(--search-highlight));
+  border-radius: 6px;
}

[AprilSylph]

Not sure how I feel about the rounded highlight. TBH I want to go all-in on the squareness and push up against the viewport edges but I don't remember how the padding is set up and don't want to either 1) resort to negative margins or 2) accidentally break something

I guess there aren't that many preference types to check and update, but still, I would rather review that as a separate PR.