Hello,
Ok… This one is on the “nice to have” side and won’t go anywhere on the priority list I guess 
In DEVONthink for Mac, user can set images for group icons, it’s nice, enhance visibility, extend customization from user perspective, blah-blah…
And surprisingly, DTTG render those icons upon sync’ing with such “fine tuned” DB. 
But as an iOS user (no more Mac since 1 year…), I wish I could customise my Groups appeal with DTTG.