I am also getting the unsupported operand type in custom-css.php file. This is on a fresh installation.
It is a shame that the support team have not resolved this and it is a bit worrying as I have just purchased the plus theme.
to resolve it, I removed the + 20 on line 40 of the custom-css.php file. I expect that this will return when there is an update. the orignal line was:
$evolve_min_width_px = $evolve_width_px + 20;
it now reads:
$evolve_min_width_px = $evolve_width_px;