Had an idea for a workaround to Freeform’s lack of dark mode. I did the below on a Mac, but you could probably do it on iPhone or iPad (just not as easily), and of course use the new template on whatever device.
- In Freeform, create a new board. Name it something like “Dark Template”.
- Change the zoom level to 10% or 25%.
- Insert a rectangle shape. Using the resizing handles, expand it to more or less fill the view space.
- With the rectangle selected, change its color to black (or whatever Dark Mode-like shade you want). Easiest way is to hit Cmd-Shift-C to bring up the color picker.
- Again with the rectangle selected, lock the shape (Cmd-L or right-click).
- Change the zoom level to 100%.
And that’s it! Now everything else you do will be on top of that locked black background.
Back in the sidebar boards list, use right click or the heart icon at far left to favorite the file. When you want to create a new dark board, just select the template and hit Cmd-Shift-D or right click to duplicate it.
You’ll note the toolbar at the top is still white, unfortunately, unless you go full screen — and then it’s dark. 
Hopefully Apple will provide proper dark mode support in the not-too-distant future.