Black theme for OLED displays, and some more theme color options?

[a1studmuffin]

I saw in a previous commit that there was once a Black theme alongside Light and Dark. This would be great to reinstate for OLED displays (and just for aesthetic preference). I imagine it would effectively look the same as Dark theme but with true black (#000000) as the background.

On a similar note it would be nice to have a greyscale theme color option in addition to Green and Pink.

If this isn't high on the priority list, I'm happy to have a go at putting together a PR, just let me know. Thanks!

[dessalines]

Feel free to add as many themes as you like! jetpack compose handles material3 themes really well, so its nbd to add these.