Rectangle Selecting color area

Build 2019.1.3539 now lets you customize the appearance using *_selectionrectangle class ("scheduler_default_selectionrectangle" for the default theme).

The default definition looks like this:

.scheduler_default_selectionrectangle {
  background-color: #0000ff; 
  border: 1px solid #000033; 
  opacity: 0.4;
}