You should apply the theme using Theme property of the <DayPilot:DayPilotScheduler> tag like this:
<DayPilot:DayPilotScheduler
Theme="scheduler_8"
...
/>
Please note that you need to include the theme stylesheet:
<link type="text/css" href='Themes/scheduler_8.css' rel="stylesheet" />
You can see a demo here (the source code of this demo page is in the download package):
http://www.daypilot.org/demo/lite/Scheduler/Theme8.aspx