In this demo the kalmstrom.com CEO Peter Kalmstrom shows how to start using CSS on your SharePoint 2010 pages and explains how to overcome the tricky part of getting it to stay.
This demo is rather old, but the method Peter shows will still work if you use SharePoint Designer 2013 and the classic SharePoint user interface. (The modern interface has no Content Editor web part.)
SharePoint Designer is the premium tool for SharePoint configuration, but if you create a new css file and then simply add a link to it in an .aspx file, SharePoint does not save it.
Peter shows how to get around this by editing the SharePoint page in the web browser and using the Content Editor web part.