Bundled Themes: Twenty Twenty block editor inserter is missing the `+`.

Fixes the disappearing `+` in the block editor when using the latest version of the Gutenberg plugin, by adding support for the native Gutenberg editor styles and removing `color: inherit;` from Twenty Twenty's style.css.

Props nrqsnchz, .
Fixes #49610.
Built from https://develop.svn.wordpress.org/trunk@47607


git-svn-id: https://core.svn.wordpress.org/trunk@47382 1a063a9b-81f0-0310-95a4-ce76da25c4cd