You can try out some other dark themes as well ... you can get it from the Microsoft Store. Black NotePad is a portable Notepad alternative for Windows 10 that comes with dark mode functionality.

There have been numerous reports on support forums, particularly Windows 10 PC users, that some apps’ background on the Start Menu tile is grey in contrast to the system Dark Mode theme.

If you want a proper feature-packed tool for managing dark and light modes in Windows 11 and 10 ... auto theme switching or a toggle in Action Center. It looks like Microsoft needs some time ...