LESS extends CSS with dynamic behavior such as variables, mixins, operations and functions:
Less for server-side compiler for .Net:
Syntax highlighting of '.less" files in visual studio:
(do not use the same names for your .css and .less files cause the vs extention will compile the .less into .css)
Quick online testing:
While LESS can be used together with bootstrap, you could also go for a Sass, Compass & Foundation combination.