I am trying to customize my theme in Opera. In the help section there is the following statement: " Select Themes from the main menu." There is no "Themes" in my main menu. How can I restore that functionality?

The heading for "Themes" (and ancillary functions) goes away when I close Opera and then open it again. If I reinstall the program, Themes appears again; however, upon closing and reopening the program, Themes disappears. Is this normal? Can it be fixed?