MacOS Dark Mode toggles back to Light Mode
I resolved this issue. Turns out I have an app installed that changes the color temperature of my screen (flux). This app seems to be toggling Dark mode back to light mode because it detects its day time. I turned the App off and this solved the problem.

Edit:
Turns out the Flux option Color Effect --> OS X Dark Theme at sunset was the root cause. I toggled this off and now I am using flux and Dark mode!