Visual tweaks suggested by @mariusz

Requires UX review by @skinny97214

Multiple CSS tweaks collected in one place, see original PRs for screenshots:

* #3224 Improve the .tour button on login.persona.org homepage
* #3223 Improve readability of .nav links in header nav

Huge props to @mariusz for submitting these, and to @ozten for suggesting we stash 'em in a branch :surfer: