The problem I was talking about is related but distinct. It concerns the Input and Output templates. With those there can't even be a single colon, since then lines other than the first end up outside of the box. If you write
{{Input|1=1. line
2. line}}
you get
1. line 2. line
but precede it with a single colon like this
:{{Input|1=1. line
2. line}}
and you get
1. line
Yes, that seems to be so. We could easily get some indent into the style for the templates, I think. Do you think it's worth while? It's relatively rare that the full width is used anyway, so I don't see that it would create too much problem.
I think that the framing and background colour makes input and output stand out nicely. On the other hand some writers seems to prefer indentation as well, and maybe it is a bit more stylish. If it can be done without causing problems, it may be a good idea.