Writing Plugin

Fonts, size, and color are determined by the theme. You can update those things in a theme without touching the theme code by using the WordPress.com Custom CSS plugin:
http://wordpress.org/extend/plugins/safecss/

You’ll need to know about CSS to use that plugin and to make the changes you’re asking about. Here is a tutorial to help you!
http://www.htmldog.com/guides/cssbeginner/