Colours are too dark when using a dark theme

[Atemu]

Here's how magit-delta looks in spacemacs using the spacemacs-dark theme:

This is how it looks in macOS Terminal.app:

As you can see, the colours are much darker and not true to the original. (Especially the green/red background colour.)

As part of troubleshooting, I removed --true-colour always from the magit-delta-delta-args which did greatly help the background colours but the syntax highlighting is still a little off.

I've also noticed that the background colourisation doesn't go all the way to the end of the line in magit while it does in the terminal. Why is that?