For example, users have reported when adding users in Moodle 3.3 using the Nonzero theme, the “Enroll Users” window appears transparent.
While it may be difficult to see, the “Enroll users” function still works as normal. Be advised, however, that these design quirks may appear in other aspects of the theme, as well as in other older themes.
While ignoring some of these visual quirks is one option, you may also consider trying a different theme. The new “Boost” Theme features a clean design that maximizes screen space and includes dynamic navigation tools such as a left sidebar that may be toggled on or off.