Interface themes allow to modify the default WinRAR look by replacing it with alternative set of graphics. Themes are distributed as RAR archives having ".theme.rar" file extension. You can download them from "Themes" section of WinRAR home page. To install a new theme, just open the theme file in WinRAR like opening a usual RAR archive, for example, double click on its icon. Then confirm installation in the dialog displayed by WinRAR.
List of installed themes is available in submenu "Themes" of menu Options. Select any list item to activate the corresponding theme. This menu also contains commands "Organize themes…" providing theme management functions and "Get more themes…" loading themes web page.
Theme files are stored in Themes subfolder of %APPDATA%\WinRAR folder.