The easiest thing I found, which is not perfect (been too lazy to make a full style mod) is to get the Stylus plugin for Chrome, and the begin playing around with making a custom stylesheet for the ebay domain.
I got a huge difference just by playing around on the global css level using:
* {background:black !important; color:white !important;}
Ex:
https://i.imgur.com/8qT2yFW.png
