I think the “usual” way is to create a custom app, modify the css in that, and it overwrites the standard desk theme.
There are a few themes available on Github, one also has several options, which might be worth a look at modifying to make the theme you need.
Has a slight issue with the gray text not being contrasted well enough to the black, for my eyes anyway.
Very colourful and multiple schemes available.
Tasteful dark pink, light theme