here's something I put together making a few missing icons with GIMP. It's a new arc theme for claws-mail based around horst3180's arc-icon-theme
. I have submitted it to the claws-mail site administrator and the first build of it has been included in their git already (v1.00). what you see here is 1.01 and I've sent the revisions in accordingly. as and when they update git again and the website in due course it will be available for download.
I like claws because it can be as simple or complex as needed. there are other themes available too which vastly improve on the default icon set.