I'm trying to customize my IntelliJ editor and have been unable to find the config that corresponds to the background color for the file tree explorer. This same color is used throughout the editor (popups, menu bars etc) and would be nice to be able to change w/ out choosing an entirely different theme.
It can be manipulated by themes but I can't find it anywhere in settings -> appearance & behavior or settings -> editor -> color scheme etc.
Project view background color depends on the theme selected and cannot be changed without changing the theme itself. You can find that information at JetBrains' bug tracker (in several last comments):
IDEA-132299 Allow custom colors for the built-in UI themes (LAFs Darcula and Default):
Now you can develop your own theme and editor color scheme. It's been announced recently in the JetBrains blog: https://blog.jetbrains.com/idea/2019/03/brighten-up-your-day-add-color-to-intellij-idea/ So please follow the instructions in the article and create your own colors.
Certain folders and files will have different colors based on the Scopes.
Also you may use another GUI Theme. You can download custom dark themes in JetBrains' plugins repository. For example "One dark theme" took 2nd place in JetBrains' theme contest.
On contest's page you could find additional 30 dark themes: