There's no way to query by rank directly, but you can order your results by theme rank first, and then by feature rank in a WSAPI query:
order=Parent.Parent.DragAndDropRank ASC,DragAndDropRank ASC
Unfortunately there's no way to specify a sort order like that in the custom list app though. You'd probably need to write a little custom app based on the Custom List app and include those additional sort fields. Are you familiar with writing apps at all?