For testing your development work in CSS, you'll need to compile the .less files to CSS. Make sure you have dependencies that LESS requires, including fabric, node, and lessc. Follow the below steps to compile the .less files:
python setup.py css
Alternatively, you can:
$ cd ipython/IPython/html
$ fab css
[localhost] local: components/less.js/bin/lessc -x style/style.less style/style.min.css
[localhost] local: components/less.js/bin/lessc -x style/ipython.less style/ipython.min.css
Done