Some remarks: A 'theme' does not only consists of (image) files, but also involves abstract things that cannot be put in a directory (colors, linewidths, highlight modes). If we want to distribute (or have users distribute) themes as downloadable entities, the settings should be supplied in some kind iof file. An ordinary settings file would be perfect for the job, as it can contain every existing option. So there would be no need to specify (and think about) what settings can be part of a theme, and which not. The designer would have total freedom.
I don't see the need for theme distributions to cram everything in a single directory. This would be a very un-Linux-like solution. Better would be to have piece sets in one directory, board-square textures in another, board rims in a third, etc. It should be easy enough to zip all components into an archive from the default XBoard data-dir. E.g. have the theme archive contain pieces/<piece_theme>/*.svg with one svg file for each piece type, two textures/*.png (or .svg) for light and dark squares, and a borders/<theme>.png for a rim around the board. By organizing the files by purpose, it is much easier for the user to browse for alternatives (when the file browser is made to start at the currently selected item) when he does want to create his own theme 'a la carte'. Especially for textures this is a great way to do it, as modern file browsers typically show you a preview of the contents if an image file as thumbnail icon. There is no need to separate svg from png resources, as the same XBoard options or dialog entries ae used for both; it would just hinder browsing through the possible alternatives. Current implementation of themes in WinBoard is that a theme is a collection of ordinary command-line options, saved on one line of a new, multi-line -themeNames option. In complete analogy with an 'installed engine' being a collection of options specifying the engine binary and engine-related settings through command-line options. The problems of theme and engine management are in fact similar to a quite high degree.
