Right now we take the separator background color and compute the opposite luma to create a base for the separator, then blend that 30% with the background.
Instead, we should use the background tab text color as the base, and blend 46% against the background, or enough to maintain a 1.84 contrast ratio, whichever is greater.
Merge justification is that for custom themes, the new tab separators in M69 could previously be colors that weren't really related to the theme at all (e.g. we got blueish separators for a black and red theme that's one of the top five Chrome themes). The new separators should be related to the tab text colors and thus look less broken (on that same theme, the separators are now red like the text).
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