The person who built my funkis dollhouse never bothered about the front panel, the "fourth wall" that you take away to look inside the house. It wasn't painted or wallpapered, and it didn't look neat. Of course you never look at it from the inside, but I was irritated.
I didn't have the same wallpaper I used inside the house, and it wouldn't have looked right anyway. I didn't have to think whether it matched the rooms since nobody would ever see it when the panel is closed. I simply took more sheets from the wonderful pad I had bought at the hobby shop and papered the ground and upper floor with different patterns. It instantly looked much better, but still unfinished.
I cut window and door frames from plain white paper, and I used the striped paper to make a border. The mirror frame came with the IKEA furniture; I painted it gold and put in a piece of tin foil. The metal strip over the door holds the panel against a magnet, so I cannot do anything about it.
I will have curtains on the upper floor, but the ground floor is a shop so I will need to figure out how to make a shop display without making the panel too heavy. I may also paint the door in a more cheerful colour.