Audit the CSS

[afercia]

Noticed while working on #833

The state of the CSS could be improved.

• there are duplicated rules between the plugin and the yoast-components: the plugin gets unnecessary, redundant, CSS
• the preview of the Wizard in the standalone app looks different from the Wizard in the plugin, which doesn't help in testing and in new development
• also, there shouldn't be hardcoded colors in the yoast-components (only colors from the palette should be used)

See also #666