Customize Thickbox background color?

[colorsofwind]

My shop's background color is not white, but some specific color. The "large" size product image defined in my shop is 360x360 and thickbox size is 900x900.

When I try to upload an image size of 180x180, I see it looks like this way as shown below:

 

I downloaded this main product image and thick box image, see they are changed to size 360x360 and 900x900. So I think, when I upload this product, those 180x180 images were processed to become 360x360 and 900x900 by adding extra pixels, those pixels are in white.

 

Those uploaded pictures of 900x900 size are normal with white edge:

 

I wonder if there is a way to change those additional white pixels to a customized color? so that those ugly white edge in the processed image will disappear?

 

Thanks a lot!