I have found that with 4.0.2 if I save a custom preset on the themes the site will crash and give me a white screen. I then have to go in the phpmyadmin and reset it back to default.
I see something similar here:
Blank white page - Questions - CS-Cart Forums
Any idea what is going on?
Would guess it's a bug and you should post it to the bugtracker (to make it public) and then also submit a helpdesk ticket (they will probably give you an immediate solution).