suggestion of the chang design detail

From what I understand, this patch modifies the default look of chord diagrams. It would be nice to be able to change additional parameters like the background color, for example to have a wood-colored fretboard, but I want to keep the default look generic so that it fits with more sites.

If you do want to customize how chords look without hacking the guitar_diagram module, I would suggest overriding the theme_guitar_diagram_chord() function and call your own image generating function instead of guitar_diagram_create_diagram().