You can change your current theme to a new by going to the Theme tab and selecting Change Theme.
When you scroll over a theme you are given the option to Choose or Preview. You can also click on the theme to view more information. If you like the theme you can choose it, if you don't you can Cancel and look through some of the other themes.
Once you have chosen a theme, you can manage aspects of its design under the Theme tab. You can also change the theme’s color (depending on which theme you’re using) and change fonts.