Use better field types for FlamingoTheme variables

Example: the field "Buttons > @btn-font-weight" is a simple text field. But actually, the only possible values are:
http://www.w3schools.com/cssref/pr_font_weight.asp

So, we should use a static list instead, or at least a <select> box for the display.

It was also possible to type a less variable. Maybe it would more user friendly to only allow that in the @lessCode field.