DoubleCAD User Manual : Components of the UI

The main area of the DoubleCAD screen is the drawing area. When creating and inserting 2D and 3D objects, you typically work in Model Space, and when putting your model on paper, Paper Space is used.
By default, the background color is white, but you can change that by selecting Options / Background Color and choosing another color.
Nearly all components of the screen can be customized. See Customizing the UI.