Modifying the header row highlight style when crosshairType="Header"

You can change the crosshair appearance using CSS. The default built-in theme ("scheduler_default") uses the following CSS:

.scheduler_default_crosshair_vertical, .scheduler_default_crosshair_horizontal, .scheduler_default_crosshair_left, .scheduler_default_crosshair_top {
  background-color: gray; opacity: 0.2; filter: alpha(opacity=20)
}

For crosshairType="Header" you need to change .scheduler_default_crosshair_left and .scheduler_default_crosshair_top.

You can override the default theme by adding a more specific selector, e.g. using the Scheduler placeholder id:

#dp .scheduler_default_crosshair_left, #dp .scheduler_default_crosshair_top {
  color: red;
}

Another option is to generate a new CSS them using the Theme Designer and modify the output CSS:
https://themes.daypilot.org/