The styling of the Manager can be tweaked to your liking, there is a base option for the theme that allows you to switch between light or dark mode (or auto based on browser preference), using the Theme input on the Server Options page.
You can also control the styling of the Manager in much more detail by using Custom CSS, which allows you to customise the Manager by injecting your own CSS. You can read more about this on the AC Manager Theme Wiki page (This guide is referencing our AC Server Manager, but the system works in the same way for ACCSM).