I would like it to be red on the left, and green on the right.
I tried to change the DiffDelete highlight, but that is for the greyed out area opposite the green highlighted text. And when I change the DiffAdd highlight, both sides change! I know that this is possible, this setup pulls it off:
I believe the theme used there is Tokyo Night, but this isn't accomplished in the theme, when I tried it, it had the same behavior- green on both sides. How can I accomplish this?