dark mode of iris-hep card for projects

[cranmer]

The IRIS-HEP logo is used as a default image for projects without logos, but it is kind of ugly in dark mode.

[henryiii]

The problem is we force a white background on all images, since some logos don't support dark mode. If we standardize on #808080 instead of using black, then we can make the background transparent and all the logos will look better.