Delta looks best if your terminal application supports 24 bit colors. See https://github.com/termstandard/colors#readme. For example, on MacOS, iTerm2 supports 24-bit colors but Terminal.app does not.
If your terminal application does not support 24-bit color, delta will still work, by automatically choosing the closest color from those available. See the
Colors section of the help output below.
If 24-bit color is supported by your terminal emulator, then it should have set the
COLORTERM env var to the value
24bit). If necessary, you can explicitly enable true color, either by using
--true-color=always or by adding the following to your configuration file:
[delta] true-color = always