Use hover css variable for primary buttons

[juliusknorr]

This should fix some buttons becoming to bright on hover/focus when using the server styles and add a missing focus border.

Couldn't find a button quickly but setting one of the firstrunwizard buttons to primary for comparison (on focus):

Before:

After:

[juliusknorr]

@jancborchardt @marcoambrosini What would be a proper focus feedback for a primary button on white background? We mostly use a 2px border now on other elements, but for that button it is not really visible from my perspective.

Edit: Nevermind just found that the button component uses the main text color then.