I am running Dopus 13 and was trying out the new Cloud Storage option available for the Folder Tree.
I run an OwnCloud instance which is not recognized by Dopus by default as a Cloud Storage provider. So I added it to the "Special Folders" list via Preferences>Special Folders>Other cloud Folders. When I added it in the Preferences pane, it shows the custom folder icon:
If Explorer shows the same icon, that's the folder's icon. It may be possible to change it (in both Explorer and Opus) via the Properties dialog, Customize tab, depending on the type of folder.
If Explorer shows a different icon, try turning this one in Opus, then open a new window to test the change:
However, Dopus shows the standard folder icon in the sidebar and not a custom icon. It's worth noting that Dopus does show the correct icon in the detail view though: