VSCode resets theme when switching tabs

Occurs when simply switching, or opening / closing a tab, and does not revert if "undoing" the tab change. Below gif's on a freshly reinstalled VSCode, though I didn't erase all cache, preferences, etc. when uninstalling.

What is the problem, and how to fix?

Solution

File -> Preferences -> Color Theme apparently doesn't change global settings, nor do I know what exactly it changes - but I circumvented the problem by changing the "workbench.colorTheme" and "theme-by-language.themes" values in settings.json to the same color theme name.

I found settings.json via File -> Preferences -> Extensions -> JSON -> Edit in settings.json