f you are expecting to be able to click a button and change fonts throughout your blog then it’s not that easily done. There are 4 ways to change fonts on wordpress.com blogs.
(1) Changing font colors and size on posts or pages either post by post or page by page http://en.support.wordpress.com/editors/font-size-color/
(2) Changing font colors using the #4 icon http://en.support.wordpress.com/visual-editor/#row-2
(3) Changing font families using TypeKit Fonts http://typekit.assistly.com/customer/portal/articles/6840-using-typekit-with-wordpress-com
(4) Changing font families, colors & sizes by purchasing an annually renewable CSS editing upgrade and doing the work yourself
Also note you may not like the fonts supplied your wordpress.com theme but it’s important to know that regardless which fonts you may wish to choose to jazz up your blog with, your browser and your visitors’ browsers can only use the fonts installed in the same computer. Obviously not every visitor to your blog will have the same fonts installed in their computers as you do on yours, so you can immediately see that problems will arise unless you choose to use common fonts.
If a reader does not have a specific font installed on his / her computer, the browser will determine which font is displayed. There are a very limited number of fonts (in English/Western languages) which are common for most operating systems/computers/browsers.
That being said, do be aware that we can all increase the font size of any webpage we are reading in our browsers. > http://onecoolsitebloggingtips.com/2010/01/19/how-to-change-font-text-size-in-your-browser/