Love that icons show on the sidebar now! 
What seems a small glitch: Icons don’t show, when the color is not defined. E.g. on this screenshot the first category has an icon defined, but no color. Then the icon is not shown:
The downside to defining all icons with colors is that icons then won’t show with inherited colors on the list views (accent when the topic is unread/ dimmed down when the topic is read). E.g. here the Announcements icon shows with inherited color, while as the icon with defined color always stays the same:
I think icons should just show on the sidebar with the default icon color for the sidebar. Then it’s up to the user if they want to define a static color or not.