Custom GUI Color

I do not think it is possible, but it can be made invisible.

Well, trmhtk2 put it very bluntly, but yeah he's kind of correct. It's a lot of steps, but I believe you can do this. What I recommend first is either going through Google and finding a picture of some default GUI or just screenshotting the picture of GUI that they give you and cropping it down and using a picture editor like GIMP to get rid of the extra stuff. Anyway, then you'll want to make the GUI invisible. Go into something like the default windows Paint (what I use) and make the GUI whatever color you want. You'll need to go into GIMP or just an online picture editor again and use a wand tool to take out the white parts and make it transparent. Then you'll need to import the picture to your GUI. Wahlah. You might need to delete your text and then put it over the new picture tho. Hope this helped. I definitely don't like the default GUI either, so I just usually use a picture of a pixel art scroll to make it look classy. 

It's actually also possible to just change the texture of the GUI in the resources tab