The new Outlook app for Windows 11 includes two color modes, dark and light, which you can switch between manually or automatically. To change the Outlook app color mode, use these steps: Click ...

On the General tab, under User interface options, select the option Optimize for best appearance. Note that the bug only affects customers with Windows 11 version 24H2.

To fix Interface not registered error 0x80040155 in Outlook, Excel, etc; register this DLL file or repair or reinstall Office. More suggestions here.