Use a special icon for a configured Archive folder
Evolution version: 3.28.5-0ubuntu0.18.04.1
I'm using Evolution on Ubuntu 18.04 with the Ubunut-mono-dark icon theme. This issue is still present when the Adwaita icon theme is in use. See the screenshots below:
This is visible in the side bar of the mail view in my instance of Evolution.
data/icons folder seems to contain a small filing cabinet icon called
hicolor_actions_16x16_mail-archive.png. I'm not sure if this helps here, but I thought I might as well point it out.