Darken hard-to-read positive, negative, and neutral selection colors

These colors were previously using the same values as in other contexts,
but this was not appropriate for the Selection set, because contrast and
readability were low. This commit addresses that by making the colors
darker just for the Selection color set. Now they are much more
readable.

This change is made to all four of the Breeze* color schemes, as they
all suffered from the issue.

BUG: 406239
FIXED-IN: 5.23