Allow theme change by URL
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= (followed by the theme name you wish to change to) depending on where you are at the time you want to swap to another theme.
For Example: If you are at the front page of your site that add ?theme=
after clicking the Enter tab on your keyboard. this will change from your present theme to the Essential theme layout, providing or course that you have that theme installed in your Moodle site.
Alternatively if you are in a course page then add &theme=
will change theme to Essential after clicking the Enter tab on your keyboard.