Bundled Themes: Twenty Twenty uses overly specific selector for button background.

Fixes the issue by removing `:not(.toogle)` from the `button` selector, as it is not needed.

Props flixos90.
Fixes #50271.
Built from https://develop.svn.wordpress.org/trunk@47933


git-svn-id: https://core.svn.wordpress.org/trunk@47706 1a063a9b-81f0-0310-95a4-ce76da25c4cd