I am not able to apply per defined css like scheduler_8 even if the cssonly option is true for scheduler.I am using nuget package of daypilot version 5.0.303.0 and .net 4.0 framework.
You should apply the theme using Theme property of the <DayPilot:DayPilotScheduler> tag like this:
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):