Theme Magic is a powerful tool for making non destructive, real time changes to the style of your Joomla template via the T3 template's administration interface.
Admintools has a feature that allows the tightening of behaviour on specific file types on your server via creating a custom htaccess file. This is an incredibly powerful tool for improving the security and performance of your Joomla site.
Theme Magic uses LESS files to make the real time changes to the design however if you are using Admintools then you need to make an exception to these files in the admintools interface.
If you do not make an exception for less files in the admintools configuration then Theme Magic will not work as expected.
In the Admintools htaccess generator configuration area you need to add the less file type to the list of allowed files as per the screenshot below:
To fix: Go to Components > Admin Tools > Web Application Firewall > Configure WAF > Visual Fingerprinting Protection and set "Allow Site Templates" to "Yes".