Curtains

What is the origin/meaning of " That will be curtains for you" Is it related to the theater?

Yes. The final stage direction - what happens at the very end - is traditionally 'Curtain': at least it was in the days when pretty much all theatre was of the proscenium arch type and had a stage and curtains.

So, by extension, 'curtains' means the end, death, or other finality.

DFG