I think the stuff I have is 90%. But nail polish remover is 70% Acetone. Yes, it’s quite flammable. But not much more than many other household chemicals. There isn’t that big a difference between the flammability of 70% and 90% and few people set themselves on fire doing their nails. Even while smoking cigarettes. 
When bonding Acrylic with Acetone, you literally use just a few drops. If the mating surfaces are relatively flush, the Acetone quickly wicks along the seam and any place the two surfaces are in physical contact (or just about), they will bond together. Only a tiny amount of Acetone is required, you’re not softening the entire thickness of the Acrylic, just the surfaces.
I’d say your risk is no worse than if you doled out a few drops of gasoline on to your workbench.
Note, this is quite a bit different than using Acetone to finish a ABS FDM print (which I do). For that, the Acetone has to be heated in to a “thick” vapor. That’s a serious risk and should only be attempted if you know what you’re doing. I would say the risk here is about the same as cooking Meth. (not that I know how to cook Meth).