Browser-laptop: Fixed white tab text on dark tab background which is difficult to read

[bradleyrichter]

Old title: white tab text on dark tab background is difficult to read https://github.com/brave/browser-laptop/issues/8115#event-1056059694

Test plan

on macOS only

1. Ensure theme colors are shown for tabs
2. Visit https://www.theatlantic.com/
3. Confirm the text is easy enough to read / that it matches the after spec (below)

Original issue description

The font weight may need to change for the white tab text label case.

[bradleyrichter]

@cezaraugusto I found a solution:

before (on Mac):

after (on Mac):

So, the weight should be 600, only for the selected tab which is displaying white text based on the BG color threshold.

This was tested to solve the mac issue and had no negative effect on Windows although the improvement is barely noticeable.

cc @bsclifton