How to fix the missing CSS/style error on the addon modules page in WHMCS

During template installation if your WHMCS addon modules page (under setup > addon modules) is showing without any styling/CSS it is generally caused by not meeting the minimum PHP memory limit WHMCS requires (128mb).

Go to WHMCS admin > help > health & updates and you will see a PHP memory limit warning.

Once you update your PHP memory limit via your hosting account it will fix the addon modules page error.