Less styling updates

This adds more variables to the LESS file so that theming is better. It removes hardcoded colors (eg white) and uses the Bootstrap variable. This is necessary when using a new theme such as the Bootswatch themes.
This looks (almost!) the same with the sample provided, but makes a massive difference if using a different theme (for example a dark background theme is almost unusable with the initial LESS file).
This is tested against the less files provided with 1.2.3, and also the latest Bootstrap (3.1.1) with the Bootswatch slate theme 3.2.0.

Unfortunately due to a format change it shows as though there are a lot more changes than there are - sorry.