Uff, done.
So added clever delete function that should really delete only content, not anymore needed by any other installed theme.
As a bonus, I created a function that validates, that really all necessary resources are available.
Test on your side (after a backup of themes) is of course welcome. If anyone will be interested in improving this "validate" function, let me know. Currently, it only checks resources defined as "file:/" in xml file.
@john_percy
because Locus Map does not use full system offered by MapsForge library, but still try to render map tiles and draw them in own system, I'm worried, this does not have a simple solution.
100% scaling ... hmm, there is no longer anything like "100%" scale as before. Anyway in top toolbar may be still visible, the scale of the current map. So by fingers, app snaps little to this 100 %. Then you may zoom by buttons by predefined step that should be same as one zoom in vector maps.