ColorPicker: Replace hardcoded "blue" with theme color

[mirka]

Description

ColorPicker used a hardcoded blue color for its input prefixes, which can look quite random when users have different admin theme colors set. This PR replaces the hardcoded color with the UI theme color.

How has this been tested?

• Storybook
• In wp-admin Site Editor with a random admin theme color set

Screenshots

Before	After

Types of changes

Bug fix (non-breaking change which fixes an issue)

Checklist:

• My code is tested.
• My code follows the WordPress code style.
• My code follows the accessibility standards.
• I've tested my changes with keyboard and screen readers.
• My code has proper inline documentation.
• I've included developer documentation if appropriate.
• I've updated all React Native files affected by any refactorings/renamings in this PR (please manually search all *.native.js files for terms that need renaming or removal).

[ciampo]

Code-wise, the changes LGTM!

The highlighted text in the color picker follows the selected theme:

color-picker-theme-color.mp4

Before merging, though, I'd like to get a blessing from @jasmussen too (thank you, Joen!)

[jasmussen]

Code looks good, thank you! Definitely use the theme color for this in similar cases as well. I do suspect some time in the future, we'll want to revisit some of those color schemes for contrast and such, but that's entirely separate.

Note that I wasn't actually able to test this, I got an error:

But I'm almost convinced that error was present in trunk at the time (14 days ago), and will be fixed if the PR is rebased.

[ciampo]

Thank you for your review @jasmussen ! I mainly wanted to make sure that it's fine to use the admin theme color in this scenario :)

Note that I wasn't actually able to test this, I got an error:

I don't think this PR could have introduced this error, since it's a very simple JS (CSS) change. FWIW, I carried my tests via this link

[jasmussen]

Yep, I recall those errors happening to unrelated branches. This looks good.