The colors for that particular styling of the theme are part CSS and part background image. That will probably not mean much to you at this time and based on the fact that you asked how to put more buttons other than Home it looks like you are REAL new to this.
In order to be able to help you effectively it will be necessary for you to get a lot more familiar with WordPress and CSS. For WordPress information you can go to WordPress.org
or seek out tutorials from various sites. For information and tutorials on CSS you can go to w3schools.com
For the Atahualpa theme the best way to learn it and WordPress in the process is to install a test site and start playing with the settings. Being a test site it won't matter if you "break" it. The settings in Atahualpa have lots of CSS information and tips.
By the way buttons are added when you add new pages or if you are using WP 3 you create them with the menu maker.
I hope you will stick with this because once things start making sense you will have lots of fun.