An option in Edit Options: "If you want to use a custom member created Bootstrap Themegen style override (in the form USERID.X), enter it here:"

I assume this is some type of coding to change the look of profile? I'd like to know more about this option and how to do it if anyone has any information.

It's a feature that's only available to creators (ie people who have uploaded mods to the site).Â* It's not available to normal members.