You can define a custom background for the structure editor in the Design environment. To replace the standard background:
1) Make sure you have a picture that you want to use as the new background in the clipboard.
2) Click in the gray area of the structure window.
3) Paste the picture (Ctrl + V on Windows or Command + V on Mac OS).
Your picture is now used as the new background.
Note: Once you have done this, you cannot revert back to the gray picture with the gears.