Hi!
Those files are only pre-created stylesheet files, only some of them are used and are generated dynamically. I don’t recommend editing them, because after saving the plugin instance they are re-generated.
In your case I would recommend to rather use one of the site theme CSS files. They usually have a custom.css somewhere in the template folder. I used to do this, it’s the safest and the best way – because then I only need to backup one css file.
There is not really a better solution to this, because if I put a custom.css file into the plugins directory, then it would be overwritten with the next update by the plugin manager automatically.