That's happened to a lot of people, even me.
The only thing I can recommend is trying a different GUI mod from the one you're currently using, but I'd wait until more people respond and see if they have a solid solution.
I had this transparent issue. What caused it is the fact that I was using the OpenGL Rendering Subsystem in Options \\ Video. I changed it back to Direct3D9 and everything went back as it was before. The background image was as it should be, solid. I should also mention that not long after I had done this, I purchased a better graphics card for my computer as I was having issues getting my new drawing tablet to work. So, I changed back to OpenGL again and the background in the chat was solid instead of transparent as it should have been.
I suggest switching back to Direct3D9 in order for it to return to it's normal appearance, if you're not using Direct3D9 already. I hope your issue is resolved soon!
I had this transparent issue. What caused it is the fact that I was using the OpenGL Rendering Subsystem in Options \\ Video. I changed it back to Direct3D9 and everything went back as it was before. The background image was as it should be, solid. I should also mention that not long after I had done this, I purchased a better graphics card for my computer as I was having issues getting my new drawing tablet to work. So, I changed back to OpenGL again and the background in the chat was solid instead of transparent as it should have been.
I suggest switching back to Direct3D9 in order for it to return to it's normal appearance, if you're not using Direct3D9 already. I hope your issue is resolved soon!
Well, wouldya look at that? It's all fixed! Gracias!