Looks nice, but hard to tell exactly without knowing dimensions and see the layout in full.
..why putting a shower curtain in would make the shower so dark, but what can I do?
1) I guess the obvious answer is: add a light source 
. Anyway, experimenting with "Incandescent light source" at various strengths and various sizes around the room should give you the perfect rendering - eventually. 2) You could also try to use a semi-transparent texture on the shower curtain. Even if that can be quite tricky to control and get right... (It is also possible to edit the general opacity of the model, but unfortunately not from within SH3D colour dialogue.)
ok
