This page links to some pictures with transparency, as well as some small pictures, smaller than 100x100 pixels, where the picture is centered in the thumbnail canvas.
We want to set the background color of the thumbnails to light-green (hex code
The config file
local/Cookbook.Mini-transparency.php has the following line of code:
$Mini['BgColor'] = array(0xdd, 0xff, 0xdd);
And here is the Mini list.
Some of these pictures have transparent pixels or alpha channels, and all of these are blended with the light-green background. Others do not have transparency.