Bistro decor challenge

[Thorstorm]

How do I know it even works?
The challenges should have a progression bar next to it.

Do I have to change the whole decor of the restaurant to Bistro or have at least one piece?

[Joney-F]

Actually, I don't really understand the decoration challenge, which is not limited to bistros. There are no progress bars or anything like that.

According to this thread, it seems possible to do this by changing all decorations to pistons, but there are also reports that it cannot be done.