Dark themes have some issues in certain gtk2 applications. Libre Office and web browsers don't handle dark themes very well. You can give a couple of them a shot and see what you think. A couple good ones are Delorean Dark, Delorean Dark Theme Stripped, and Mediterranean Dark. A google search for any of them should find them for you. Maybe also try searching Deviant Art for gtk themes. There are a lot of them. Be careful of the noobslab theme ppa. The one time I used it to install a gtk theme it wanted to install all of gnome shell. Your better off finding and installing these themes on your own.