Twenty Thirteen: Fix button block custom colors.

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.

Also make sure the button’s gradient is not applied when a custom background color is added, since it will hide it.

Props laurelfulford.

Merges [44195] into trunk.

Fixes #45431.
Built from https://develop.svn.wordpress.org/trunk@44304


git-svn-id: https://core.svn.wordpress.org/trunk@44134 1a063a9b-81f0-0310-95a4-ce76da25c4cd