Upload "xenforo_requirements.php" to the root of your XenForo installation.
Access the requirements test page by entering the following url:
XenForo System Requirements
PHP extensions: MySQLi, GD (with JPEG support)...
This is a very simple tutorial by @Caldexi that contains A few steps that will ensure your options and custom fixes are not erased!
Import a style and give it a unique name, should look like this (disable user selection):
Create a new child style and set the parent...