All I can find is documentation for changing "SVG" file colors on this page: https://www.diagrams.net/doc/faq/svg-edit-colours
But most of the icons in the shapes library are not svg files, but images. (And there is no way to search the libraries for only configurable icons).
Do I have to create a new file and change its color and add to my own library? (That's a heck of a lot of work for entire icon sets). I keep thinking I must be missing something. If someone could explain this, I bet more of us mortals could get a lot more out from this product.