For comparison, NeoLemmix implementation: The user can have a "styles.ini" file that specifically tells NL what order to put styles in; it can also set user-friendly names for them that differ from the folder names. If this file is absent, or for any style not listed in it (in this latter case, unlisted styles are always after the listed styles), the remainder are sorted alphabetically by style name. (With this being said, in NL, almost all style names are in turn prefixed with the author name though, so this essentially becomes sort-by-author. There's a few that are instead prefixed with "orig" or "ohno", as well as three that are not prefixed, two of which are special-cases - "default" and "special" - and one of which comes from an official game that only has one style - "xmas".)
NeoLemmix ships with a styles.ini file that contains most styles anyway. Here's a copy of the default.