I'm looking to create an extension for personal and possibly personal friend usage as well that simply changes a few colors that are displayed on a website I visit. I use Reddit Enhancement Suite, which has options for 'Night mode' that completely changes the color appearance of Reddit.

I would like to know how to edit or inject CSS code to this website so that the colors used in the website are what I would like them to be when I view the page. Mind you, this is of course just in my browser or in another Chrome browser using my extension.