Allow theme change by URL
From MoodleDocs
Site Administration > Appearance > Themes > Theme settings
Enabling this setting in Moodle 2.0+ themes allows you to change theme by adding either ?theme= or &theme= depending on where you are at the time you want to swap to another theme.
For Example:
will change from your present theme to the Essential theme layout.
Alternatively if you are in a course page then add this to the end of the url that you see in your browser adressbar...
- &theme=essential