Increase color contrast on default scheme admin bar hovers

[melchoyce]

It's been pointed out in #28599 that the default scheme admin bar hover color, #2ea2cc, is low-contrast. It failed when I ran it through a color contrast tool. I played with some alternate colors and came up with #45bbe6, which is a bright blue that passes with a contrast ratio of 5.72:1. See attached screenshot and patch.

[helen]

The relative luminosity (I am probably butchering my color terminology here, apologies) of the brighter blue on dark backgrounds has bothered me for a while. This looks much closer from a human visibility perspective and is much easier to read. It also passes contrast guidelines, which is a win. Testing and will commit shortly.

[helen]

In 30313:

Use a brighter blue against dark backgrounds in the admin menu and toolbar.

Not only does this pass contrast guidelines, but it also visually appears closer to the original highlight blue used against light backgrounds.

props melchoyce. fixes #30323.