The shadow highlighting the new event position during moving and resizing uses semi-transparent background.

You can customize it by modifying the _shadow and _shadow_inner classes in the theme.

.scheduler_white_shadow_inner 
{
  background-color: #666666;
  opacity: 0.5;
  filter: alpha(opacity=50);
  height: 100%;
  -moz-border-radius: 5px;
  -webkit-border-radius: 5px;
  border-radius: 5px;
}