This post will guide you through the steps to disable dark theme and restore light theme or vice-versa of Firefox developer edition.
1. Open a new tab. Enter
about:config in address bar. Below prompt will appear.

2. Click
I'II be careful, I promise! Button.
3.Search for
devedition 
4. Double click
browser.devedition.theme.enabled row.
5. Enjoy the theme you want.