You don't mention what OS you installed Grafana on. For Ubuntu/Debian, the css files are found at /usr/share/grafana/public/css. There is no built-in way to customize the appearance. You can change the CSS files but they will get overwritten if you upgrade Grafana.
You can however fork Grafana and change as much as you want to.
The instructions for building Grafana are here: http://docs.grafana.org/project/building_from_source/
The frontend files are located in the public subdirectory and when you run the Grunt build script, the CSS and Javascript files are generated in the public_gen directory. So do not make your changes in the public_gen directory as they will get overwritten.
Thanks for answering, Daniel. I am currently running on Mac OS, but I will eventually be running on Ubuntu Server. Unfortunately, the link that you have provided me is throwing a
404 - page not founderror.Sorry about that. An extra character had got added to the link. It's fixed now.