Any user with the proper permissions, including teachers with editing permissions, can change to the desired theme in order to design the course theme. Course theme overrides site themes, user themes, or session themes. In other words, if a user has set a theme and the editing teacher has set the course theme, the course theme is the one which is going to be seen instead of the one chosen by the user.
In order to allow an editing teacher to design a course theme, we must follow these steps:
Under Site administration, click on Appearance | Themes | Theme selector.
Click on Allow course themes as shown in the following screenshot:
Then scroll down the page and click on Save changes.