Button color settings overridden in Twenty-* themes.

[m-e-h]

Describe the bug
As mentioned in #11779 (comment)
The Twenty-* themes are overriding the core button styles such as .wp-block-button__link:not(.has-background). But they're doing it with an equally specific selector that also overrides the "utility" class applied for the color .has-pale-pink-background-color.

By relying on :not() to allow for color overrides, we're assuming everyone else will, as well.
Using :not() to style a button wouldn't be my first instinct.
Also discussed here #11677

To Reproduce
Steps to reproduce the behavior:

1. Add a button in Twenty Seventeen or Twenty Sixteen from the WP 5.0 Branch.
2. Color the background and text color of the button.
3. Check it out on the front end.

Additional context

• Gutenberg v4.4

[designsimply]

Testing note: check to see if it happens in the plugin independently of WP5.0-betas.

[m-e-h]

This only happens when a theme explicitly sets the color or background on .wp-block-button__link.

So it would only be a problem for the betas of the Twenty-* themes since the previous versions don't have any Gutenberg styles.

[m-e-h]

This can probably be closed with the fix in #12005
But the default themes are still going to have to adjust their CSS to allow for the Button Color settings to take affect.

[youknowriad]

Thanks @m-e-h Awesome work.