You will think me a terrible fool, but I still can't get the theme to behave.
I downloaded
https://stash.reaper.fm/theme/225/Ado...20Theme%20v1.5 and unzipped it, placing what it created, the AA 3.0 V1.5 folder, in C:\Program Files\REAPER\ColorThemes.
The AA Theme folder contained the following: 2 themefiles and two folders, "Adobe Audition 3.0" and "Splash". I took the two theme files and placed them in the root ColorThemes folder as you suggested.
So, to cut a long story short, the content of C:\Program Files\REAPER\ColorThemes is now:
a default.reapertheme, a default folder containing the default images, Adobe Audition 3.0.reapertheme, Adobe Audition 3.0 (Dark Selection).reapertheme and the AA 3.0 V1.5 folder containing "Splash" and "Adobe Audition 3.0".
Reaper now offers the two Adobe themes, and when I load them some of the colours change but certain things, such as the menu buttons, do not.
Sorry to make such a simple matter so confusing.