Add dark mode support in twenty twenty-one theme

[thelovekesh]

Summary

Fixes #6783

• Add support to toggle dark and light mode using AMP.toggleTheme component
• Add dark mode class name to $class_names[] so that dark mode class used by AMP.toggleTheme can be prevented from being tree-shaken.
• Remove the dark mode toggler button notice from the dark mode customizer.

See recorded screencast

c2011e824f.mp4

Checklist

• My code is tested and passes existing tests.
• My code follows the Engineering Guidelines (updates are often made to the guidelines, check it out periodically).

[CLAassistant]

All committers have signed the CLA.

[delawski]

The dark mode toggle is now rendered on AMP pages when Twenty Twenty-One theme is activated. The color mode preference is persisted across visits.

There's a minor difference between AMP and non-AMP versions: the mode toggle is not hidden/shown on scroll:

tt1-dark-mode-toggle.mp4

I don't think it's a blocker but if it's something relatively easy to achieve, it'd be nice to have.

[delawski]

@thelovekesh It seems that the unit tests need to be updated. Can you have a look into that?

[thelovekesh]

It seems that the unit tests need to be updated. Can you have a look into that?

@delawski Updated the test cases.

[delawski]

@westonruter I think this is now ready for your review.

[westonruter]

Thanks a lot for working on this. Great that this includes not only dark mode support for Twenty Twenty-One but also updating the style sanitizer so other themes can implement dark mode.