Improve themes, add some scss vars, add missing localization for the default theme
Summary: T427454, WARNING: Changes to external themes required!
Test Plan: ./phpunit --testsuite=Browser
Reviewers: #hkccp_developers, vanmeeuwen
Reviewed By: #hkccp_developers, vanmeeuwen
Subscribers: #hkccp_developers
Differential Revision: https://git.kolab.org/D2599