Can I change the widget to always appear in light mode?

No. BusyCal’s widgets follow your device’s system appearance (Light/Dark/Automatic).

Even if you set BusyCal itself to Light Mode or Dark Mode, the widget will still match the system.

If you want the widget to stay light, set your device appearance to Light in system settings (or adjust your system’s Automatic schedule). For changing the app’s own appearance, see How do I change dark mode settings?.