Add dark mode to Sepia theme

[justin-b-reeves]

@media (prefers-color-scheme: dark) for Sepia theme

[brentsimmons]

Let’s do this instead: make this a separate theme with a separate name, Something Dark, where the Something is whatever you choose.

Here’s why: Sepia has been around for a while, and it’s a solid bet that there are people using dark mode with current Sepia and who like the look as-is. They’d be rightly upset if Sepia changed.

Feel free to copy as much or all of Sepia as you need to make this work, and also feel free to change any part of it too on the way to creating this new theme.

Thanks!

[justin-b-reeves]

#5106