Theme color: Use dynamic variable from user preferences

[psealock]

Partially addresses #4544

Colors for buttons and various other core Gutenberg elements should get wp-admin theme colors.

Detailed test instructions:

1. Poke around the app, see button colors rendered correctly.
2. Edit your user profile

3. Change to a new color scheme.

4. See the new colors reflected.

Note

There are still some instances of colors being overriden where they shouldn't be (breadcrumbs?), some colors need to be tweaked (date range picker), and areas where overrides need to be placed (OBW). I'd like to follow up with those fixes, this PR is just the mechanism for getting G2 colors shown.

Instead of darken, lighten I used color function. See https://github.com/postcss/postcss-color-function#notes-for-former-sass-users

Changelog Note:

• Enhancement: Use Core colors for styling accents.

[becdetat]

Roll over on busy state removes the animation, it's like the :hover is given more precedence than .is-busy.

Not sure if this is due to the changes or if this is on master but the 'Dismiss this message' text goes over two lines. Possibly something for @octaedro to look at?

The checkboxes on the industry step are still misaligned, not sure if that was meant to be part of this fix:

The checkboxes and the step checks are also purple, which doesn't really 'go' with the blue buttons, is that right?

[becdetat]

Code looks good, everything looks good, I would say that my comment is out of scope...

[psealock]

Thanks @becdetat for the review.

Thanks for noting the issues you spotted in the first comment, I'll be sure to address those in the follow up.