LESS extends CSS with dynamic behavior such as variables, mixins, operations and functions:
http://lesscss.org
Less for server-side compiler for .Net:
http://www.dotlesscss.org
Syntax highlighting of '.less" files in visual studio:
http://visualstudiogallery.msdn.microsoft.com/e646c6ec-87a7-45ea-81e8-d655a3d3e73e
(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:
http://winless.org/online-less-compiler
While LESS can be used together with bootstrap, you could also go for a Sass, Compass & Foundation combination.