Gareth J Barnard, the maintainer of Essential theme which is one of the most downloaded themes in the Moodle themes database has developed a block which allows you to switch the themes easily. The block named “Theme Selector” is basically meant for theme designers who used to constantly switch themes during development.
I have tried the same on my development server and found it very useful especially if you are a theme designer as it allows you to switch different themes easily without the conventional ‘Theme selector’ or ‘Theme URL’. It requires the capability ‘allowthemechangeonurl’ core theme setting to be checked before this block can be used.
You can download the Theme Selector block from the GitHub page – https://github.com/gjb2048/moodle-block_theme_selector. You don’t need anything else like jQuery as the block now gets from core via the fully rewritten AMD module.
The block will be released in the Moodle plugins database if people will mark the original forum thread as useful. So, please make sure to mark the forum thread mentioned in the references below as useful.