It is possible to make custom backgrounds for GUIs and slots but its not possible to change the texture of the button due to no file or texture we have access to to edit in an image file.
Could we please have support for changing the way buttons look in our GUI even if its just support for texturing the button itself so we can make it match the rest of the GUI.
Here is an example:
Minecraft beacon buttons are below the GUI image.
Even if we got a seperate file with just elements for our GUI like buttons, images, and text boxes it would help customize our GUI's a bit more.