Need ability to set color of primary tab so custom themes are not ugly

[herbdool]

Describe your issue or idea

Because the primary tabs aren't a color option in Basis, it means that it's easy to make ugly looking color schemes. I'm going to be controversial and mark this as a bug (though even as a UI tweak might make it into 1.11.x?).

Steps to reproduce (if reporting a bug)

Create some color schemes like "Tropical" in this PR backdrop/backdrop#2299.

Actual behavior (if reporting a bug)

Expected behavior (if reporting a bug)

Primary tabs should be a color option so that it can be controlled better.

Tested with 1.11.x-dev

[quicksketch]

This looks good to me. With the problems noted in backdrop/backdrop#2299, this seems like the magic blending approach isn't going to cut it for Basis.

[quicksketch]

PR was minimal and reviewed by @oadaeh. Merged backdrop/backdrop#2303 into 1.x for 1.11.0. Thanks @herbdool!