I don't have the option for Gallery only HTC Background
From what I've seen on the forums, it's literally a hit or miss thing. Some people can select any background (myself included) and some people can only select HTC backgrounds. Literally have no idea why but there was a thread with a bunch of conversation over it!
On topic though, yes, I LOVE the stock messaging app, no problems, and then white/vibrant green combo with the white background is the best looking option I could think of! I wish you could just pick a custom color for each bubble though with something like a color picker, or at least a little more of selection, but I'm very satisfied! Plus you can change how many lines are previewed in the text from the list of contacts in the app, and the notifications will show you the entire message in the notification screen so I'm happy!