The "+" and "-" boxes are line drawing code inside FBReader, so themes won't help. There is definitely a bug, because if you increase the size of the library view fonts the boxes should get bigger and they don't. I have updated FBReader's
Bug Tracker to reflect this. The bug might in fact be fixed in the latest code (at least it does not happen on other machines now, if it ever did). It is easy to test for the bug: if the boxes stay small even with a large font then we know the bug is still present.