Yes, this is possible, but you might get in trouble. See this answer. Basically Github will serve your files as "raw" files. The browsers must then depend on the type
parameter, which may or may not work as it should.
Github also has a bandwidth limit so if you have a lot of visitors they might cut down on your bandwidth usage (i. e. block access to the stylesheet).
There are other problems, as outlined in the linked post. For example the lack of caching which means your visitors have to download the stylesheet every visit they make.