Shorten immutable tabbar and its separators, and blend highlight with window color not base color

!478 (merged) saw some quite negative feedback from users. This doesn't fully address that, but reduces the very tall height of the immutable tabs and their separators, reducing the button-like appearance, and blends the highlight background with the window colour rather than the base colour, slightly improving the connectedness with the contents.

@teams/vdg

State	Configuration	Properties
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After