UX refresh

Brings the style of X-Ray Goggles up to date with the other Webmaker tools, specifically:
* landing page
* toolbar
* remix dialog
* publish dialog

Addresses these tickets:
* https://bugzilla.mozilla.org/show_bug.cgi?id=895602
* https://bugzilla.mozilla.org/show_bug.cgi?id=902188
* https://bugzilla.mozilla.org/show_bug.cgi?id=902192
* https://bugzilla.mozilla.org/show_bug.cgi?id=902647