When Opus imports or upgrades over a config from an earlier major version (e.g. 11.x vs 12.x) it may move custom toolbars to backup copies and reset to the default toolbars. "Your old toolbars are still there after you upgrade" explains in some more detail and has instructions on how to turn your old ones back on.
Note that you'll probably want to copy at least some of the changes form the new defaults over, particularly the new Properties > Set Label and Status items.
Tab colors behave differently in Opus 12. If we're talking about the colors for linked tabs, those are now used to change the color of the chain icon on the tab, and not the whole tab. Tabs can also have colors assigned down their edges, as an accent, to indicate the tab itself (whether or not it is linked) or to indicate particular folders.
Some of this is demonstrated in the What's New video, where the Folder Tabs section starts at 9m10s: