Site Administration > Appearance > Themes > Theme settings
Enabling this setting allows you to change the 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 adds ?theme=
http://localhost/lms30/?theme=essential
after clicking the Enter tab on your keyboard. this will change from your present theme to the Essential theme layout, providing, of course, that you have that theme installed in your LMS site.
Alternatively, if you are in a course page then add &theme=
http://localhost/lms30/course/view.php?id=3&theme=essential
will change the theme to Essential after clicking the Enter tab on your keyboard.