I don't know why this is happening but I've had multiple people tell me they have had this issue. I've experienced it myself.
Ah, so it doesn't happen ever time? Can you provide an example link I can use to try to trigger it myself?
Also, are you using an up-to-date browser? See http://browsehappy.com/
I've tried multiple themes and am still having the same sorts of design and implementation problems I did in the past. This occurs when I tried to change anything other than use stock themes as is. These are not heavy modifications. I changed from white to black, added a header and changed the main page to fixed. Still I am having issues. I paid extra to have the custom package but am afraid at this point to even do anything with it. At this point, I'm not very happy with my choice for hosting this.
Sorry to hear you're having trouble! Maybe you've happened to try some modifications that were tough to make based on your theme choice, and if that's the case, the forums are totally here to help you out. Just make a new help request for each separate issue, and we can totally work through them. What you described, such as making a main page fixed, could be super simple in one theme while it isn't in another. So I think it's worth looking at some of the customizations you want to make in more detail.
They are there and if I click on the blackened page, the carousel opens and the images are viewable. This would contradict your diagnosis.
Aside from CSS, the galleries issue you described don't sound right to me, but I haven't seen any other similar complaints about that posted in this forum either. I'd like to investigate it if you can provide some links for me to test out.