This evening I was setting up a schedule for our church programs this weekend. I knew I had a particular background in the Themes that I wanted to use for a particular song - but could I find it - no. I searched up and down my entire thumbnails of song themes and could not find that background theme anywhere. I changed the thumbnail sizes tried again and again but that background was just not in the themes list.
Eventually, I saved the entire schedule shut down EW and restarted it then went straight to the song themes and there was that background on the 3rd row down - about where I had thought it should be all the time.
Problem is, this is not the first time this has happened. Previously I have also searched for a background theme I knew I had but it was not there. until a restart of EW.

Seems to me there is a bug in the system somewhere that hides - at random - some of the theme thumbnails
I seem to remember another post to this effect at one stage. Yes I am running EW 6 2.5.5 the current latest version.