`kitten theme` runs the themes kitten -- kitty accepts unambiguous prefixes
-- but the shim tested `$argv[1] = themes` exactly, so that spelling skipped
the capture and re-apply entirely. Before the theme-mode work that was
harmless, since nothing rewrote the automatic theme files; now the next
regeneration reverts whatever was just chosen, which is why picking a theme
stopped sticking.
Accept any prefix of "themes" at least two characters long. Verified that
themes/theme/them/th all trigger and icat still does not.
ccf4a18c86