Hello,
Unfortunately, I have to say that Task Scheduler does not currently support a native dark mode. Although Windows 11 and Windows Server 2022 allow you to enable the system dark theme. Some of the tools such as Task Scheduler, Event Viewer, Services, Device Manager are built on the Microsoft Management Console (MMC) framework. So this is a design limitation rather than a configuration issue, and there is no setting in Task Scheduler itself to change its appearance.
About Task Manager: On Windows 11, newer versions of Task Manager support dark mode and follow the system theme, so it could be use in dark mode. And on Windows Server 2022 The Task Manager included with Server 2022 is based on an older implementation and does not support dark mode, even if the desktop theme is set to dark.