v4: Miscellaneous color changes

[mdo]

This branch addresses a handful of color changes and contrast improvements. It doesn't solve everything from #19693, but I'm okay with that since the $brand- color changes aren't very good looking to me. We'll deal with that later.

• Darkens the close icon colors via opacity changes.
• Fixes $gray-light color inaccessible on white background #20952 by darkening the $gray-light value.
• In concert with the above fix, darkened $gray and $gray-dark to keep the color scheme intact.
• Darken the opacity of text in active list group items.

/cc @patrickhlauke

[Francismori7]

Sadly I did prefer the brand colors in #19693, they were more vibrant to the eye on my screens.

[mdo]

@Francismori7 I'm not saying we won't have changes to those colors; just not those exact values at this particular moment in time :).

[Francismori7]

@mdo don't worry about it!