Not kidding, Blue theme is .. 5.4 or less, Green theme 5.5
Other ways to find
For 5.5, after login in welcome screen, Lower left corner "License information"
For older versions, Click on the CUBE on the top right most corner.
Live and Learn, thanks much
I should learn to ask more specific questions. I was also interested in knowing how to determine what SP levels and patches are present in the instance. Do I just nose around in the package manager? There must be a better way.
You can determine this in the Package Manager. But as indicated previously, if you look at the license information section, it will tell you. Here's an example from my instance: Adobe CQ, Version 18.104.22.16820220 Service Pack 2