WordPress.com blog hosting offers 100+ themes to choose from, you can also take your site’s customization a step further and completely personalize any theme’s stylesheet by changing fonts, colors, borders, backgrounds, and the layout of the site using the Custom CSS upgrade.
The Custom CSS Upgrade allows you to edit your theme’s CSS (Cascading Style Sheet) and customize the appearance of your blog. CSS allows you to change fonts, colors, borders, backgrounds, and even the layout of the blog.
Custom CSS is a paid upgrade that costs $14.97 per blog, per year.
More info: http://en.support.wordpress.com/custom-css/
