Themes are located in the /themes folder. You should avoid editing the default theme. Instead, make a copy of the default folder and rename it to the name you want for your theme. Upload it to the themes folder. In Admin - Interface select the theme you want active. In Admin - Home at bottom clear the cache. Your new theme will be active. Always keep in mind that changes to a theme must be followed by clearing the cache for the changes to show in your browser. The procedure is the same for most provided themes here. The only caveat may be any core file (in main WeBid folder, PHP files) changes the author makes to make their theme work the way they want.