http://blogbettr.com/wordpress/customize-your-theme/

Customizing your WordPress theme – whether it’s free or premium – is often a daunting prospect, and often the changes you want to make which should be straightforward quickly become very complicated.

Thankfully, making simple changes — like changing the font, background or site colors — is a simple process and if you’re careful, you can even ensure the bits you do customize are protected from theme updates.

In this post I’m going to show you how to get started customizing your WordPress theme. These are the very basics, so if you’re after something more complicated check out WPShout.

Customize your theme: getting started

We’re going to be doing our customizations via the plugin Jetpack. Jetpack brings some of the features WordPress.com users are familiar with to your self-hosted WordPress install. One of those, conveniently, is a CSS Editor! This is where we’ll be doing the customizations.