Click the scroll-down arrow in the middle column and select "Add CSS." Enter your custom CSS code in the "Add Custom CSS" box. Click the "Apply to Blog" button located in the right corner of the page.

All you have to do is store your HTML, JavaScript, and CSS files, and then share a link to your starting HTML file. In fact, Google is eating its own dog food by hosting the instructions on how to ...

I'm not CSS expert, but I've spent a good amount of time hacking CSS for my own pet projects, and believe me: A little bit goes a long way, but there's plenty you can learn if you want to go deep.