In Styling tab / Themes sub-tab, you can select a specific theme for this site (from your previously uploaded Themes):

themes, img #01.1

In the Settings sub-tab you can select your slide-bar type (that includes Browse tree, Advanced search and Facets) and its position (left or right):

Managing Sites, Styling, img #02

Custom CSS and Custom JS sub-tabs allow you to add your own custom CSS and java-script for this site and make it Active or Not. Note troubleshooting tip: Activate and deactivate your custom code window one at a time to determine if your code is causing issues with the site.

customCSS, img #03

Once you are done you can press Save to return to the Sites menu or you can hit Apply and go to the Next tab.