Updates to the default admin color scheme

Over on Make/Design, @hugobaeta has posted about some subtle updates to the blues and grays of the default admin color scheme. Work on updating the admin has been completed in #31234. For those who are using those colors in their development, particularly any large areas of colors such as backgrounds, you may want to take a moment to update those values. There’s a handy guide to the changes on the post.

While we haven’t yet settled on a sensible and semantic way to provide helpers or utilities for these kinds of CSS changes, thoughts and eyes are welcome on #26691 for that.

#4-2, #bikeshed, #css, #dev-notes