If you change the background color for thumbnails in Administration / Settings / Thumbnails it won’t change the display until you reload the corresponding pictures…
Yes, it is a pain, so it is better to have it set right before uploading a number of pictures of products.
I found out by asking CS-Cart support. I have not found the data elsewhere. I imagine that could easily be changed, and should.
The GoldBarKid
It WOULD be inconvenient to reload all thumbs for a large amount of products but I have done this in styles.css around line 1402 I believe.
background-color: transparent;