You'll find the style settings right in your ACP if you click on Configuration
Depending on which style you're using there could be more or less options. Some options might just remove the rounded corners (border-radius) from avatar images, others might load a redesign of the members list. Just play around a bit to find out what you like you can always change again them later.
The purpose of these style options is to help you customize the style to some extend without having to code.
If you do have more questions, please feel free to open a new thread on Elevenfour's support forum (account needed, you can cancel any time).