You can find the theme in RW 6? Then all is ok.
Here or there!
You can search the theme in Apple OSX.
Or, you look here! Open “Go to” window in the menu, press the alt/option button, then the library will appear between User and Computer.
Then you must click the whole path.
For me this path.
For you …
/Users/your name/Library/Containers/com.realmacsoftware.rapidweaver6/Data/Library/Application Support/RapidWeaver
Or you download the theme again, from the developers website.
Which theme you are looking for?