Hello there.The templates and themes for wordpress.ORG installs are not the same versions as those for wordpress.COM blogs. WordPress.com uses modified versions of the themes so they can be run on this wpMU (wordpress multiuser blogging platform) and in some cases the fonts have been changed from those found in the wordpress.ORG versions.
1. We wordpress.com users can change font sizes and colors on our posts and pages, a post and a page at a time. http://en.support.wordpress.com/editors/font-size-color/
2. We can also use the number 4 icon in the Visual editor to to change colors > http://en.support.wordpress.com/visual-editor/#row-2
3. We can use TypeKit Fonts to change font families but not font sizes and colors throughout our blogs > Appearance > TypeKit Fonts
See > http://getook.wordpress.com/2010/01/04/typekit-guide-change-your-fonts/
4. We cannot access the templates in our wordpress.com blogs and edit the underlying PHP and HTML. However, if we choose to pay for an annually renewable CSS upgrade and undertake CSS editing then we can change font styles, colors and sizes throughout the entire blog.
This paid upgrade is NOT recommended to those who do not have at least a moderate understanding of both HTML and CSS, as there is no Staff support for it, and precious little Volunteer suppor, which means you are expected to be able to work independently.
If you are considering purchasing the upgrade, you can go to Appearance > Edit CSS. There wordpress has provided a preview function where you can try before you buy.
I spologize for not being able to be more helpful than this. Gopefully, another Volunteers may be able to assist you further.
Best wishes :)