It's possible to change the background color using CSS.
1. You can create a custom theme:
https://themes.daypilot.org/scheduler/create
2. You can override the background color specified using the default theme:
#dp .scheduler_default_message { color: #ffffff;background: #red; }
Please note that #dp is added to the selector to make it more specific so it will override the theme.
3. You can add custom CSS class to the message bar when you display the message:
dp.message("Hi there!", { cssClass: "red" });
.red.scheduler_default_message { color: #ffffff;background: #red; }