A more organized version of the stylesheet for the Duelyst subreddit.
First install Node. The are various ways to do this depending on your platform; if you're using Mac OS X then Homebrew is your best bet. For Windows, installing it with their own installer is the way to go.
After that, you want to run npm install in a terminal in the project folder to get the LESS package.
Now write some LESS code. If you're unfamiliar with how LESS works, check out their documentation.
Compile! Simply run npm run css which contains the LESS script for compiling the LESS files into a minified CSS file, which can then be pasted into the stylesheet textbox in the subreddit editing tool.