I expected that specifying theme in the Admin Panel will certainly install it for all users. However, if a registered user has set a theme in his a profile before changed theme by the administrator, then this setting does not change. I did not understand how to install theme for the guest. Cleaning caches of wiki and browsers give unpredictable results. I myself am embarrassed to reproduce the issue. So far, I realized that it is necessary to allow one theme only and set it in Admin Panel. After that, I need clean all caches of wiki and browser. Then (may be) users and guests will see same theme.