Bundled Themes: Fix button block custom colors on front end.

Simplify some of the button block’s CSS selectors in the theme, to make sure the default colors don’t override the custom colors on the front end.

This fix is applied to Twenty Sixteen, Twenty Fifteen and Twenty Fourteen. 

Props laurelfulford.

Merges [44197-44198] and [44200] into trunk.

Fixes #45428, #45429.
Built from https://develop.svn.wordpress.org/trunk@44306


git-svn-id: https://core.svn.wordpress.org/trunk@44136 1a063a9b-81f0-0310-95a4-ce76da25c4cd