Scheduler - cell mouseover time headers shaded with custom colour

Yes, you can do it using CSS. In the default (built-in) CSS theme, the following styles are used:

.scheduler_default_crosshair_vertical,
.scheduler_default_crosshair_horizontal,
.scheduler_default_crosshair_left,
.scheduler_default_crosshair_top {
  background-color: gray;
  opacity: 0.2;
}

The time header part of the crosshair is marked with scheduler_default_crosshair_top CSS class.