If you're able to run Sweet Home 3D from code source, simply change implementation of getBackgroundColor and getForegroundColor in PlanComponent class. Otherwise, this might be difficult.

That's not even close to a Dark Mode. Implementing a Dark Mode is basically implementing a theme system and that's not an easy or quick job. I haven't seen a response to my initial information request but I suspect that the first step needed is switching from ancient Swing to JavaFX. I have no idea how easy or difficult this is, the sources I found online differ very much in their options. Looking at the future, switching to JavaFX might not be a step that can be ignored.