You may select the default theme for domain owner Webmin logins. It defaults to the global Webmin default, which is the Virtualmin-Nuvola theme in Virtualmin Professional installations. You may find it useful to differentiate between colour schemes for various privilege levels, like choosing a red-tinted theme for the administrative accounts, since those accounts are most dangerous when used carelessly, while using the standard soft grey-blue tinted theme for your domain owners and resellers. Purely a matter of taste.