LESS-based theme implementation.

The idea is that for each theme, a resources/css/theme/<themename>.less
file will be created. Ideally that file merely @imports the appropriate
color file from the resources/css/theme/colors/ directory, after which it
imports resources/css/theme/theme-template.less.
This allows to reduce unnecessary code duplication.

Change-Id: I3bea7e270c3551e0ea0ac568a7ca18a5aced5cdb