I have gedit 3.6.2 and I've installed a plugin (gedit-markdown: support for Markdown language in gedit) that allows me to preview markdown text as html in a separate panel within gedit.
I want to know if it's possible to change the background color of the lower panel from white to something else like #999. My guess is that I may need to edit some css in my theme. I'm using Greybird (from "shimmer-themes" in the Software Center).
For the upper (main) panel, I use a slightly modified Cobalt color scheme.