Nope. I would not consider it theft and I wouldn’t be stupid enough to tackle an armed security guard over a piece of cardboard. Use your head for gosh sakes.
And if I was blocking an entrance to private property and preventing people from entering I would expect that my sign may be taken and I would be asked to leave. Your first amendment rights end on private property. And you have no right to block people from entering buildings.
Odds are this person with the sign isn’t even here legally so as far as I am concerned they have no rights at all if that is the case.
As you can see in this video anyone on the sidewalk wishing to enter the building was free to do so.