I’m writing markdown basically everyday. I love Visual Studio Code as an editor and i’m using Markdown Preview Enhanced as my preview tool. Now out of the box the preview window is white, and I prefer a darker theme and have set my VS Code to have a darker theme, more precisely One dark pro.
What is most annoying is the white background on the Preveiw window, it hurts my eyes having a darker theme and a white bright preview window. So lets fix this! Open your settings.json file, that holds your settings for Visual Studio, if you dont know how to do that; search for Open User Settings (JSON) in your command pallet. To open your command pallet on MAC press Command+Shift+P and Windows Ctrl+Shift+P
You will open a JSON file and here you can add a theme for Markdown Preview Enhanced Preview theme;
“markdown-preview-enhanced.previewTheme”: “one-dark.css”,
You can set the theme’s css to any of the installed theme’s you have. For me some of the options are as image below 
And by setting this I now have a dark preview window for my markdown! Got damn that is neat!