I have some information from the FAQs that also may be useful if you decide not to opt for the css upgrade. I’m offering this only to round out the full array of choices that are available to you as the others have done very well providing the options. There is one other to consider.
It’s true that you cannot change the default font your theme uses. If you want to do this anyway, you must do this all the time. This is of course is tedious but it can be done. The instructions for how to change it all the time are here http://faq.wordpress.com/2006/06/12/can-i-change-my-font-font-size/