UBB.threads styles utilize CSS coding to present a custom style to your users; the style editor can be found in the Control Panel and it'll allow you to edit, and preview, any changes to your style. Simply open the body class in the editor and make any changes to the associated CSS and save your style.
Take care to not edit any styles directly via FTP, as they will be overwritten when a style is edited via the style editor. Documentation on CSS can be found at
w3schools.com: CSS.