The script works by replacing default elements with custom ones and enables safe backup/restore.
A GUI is available here.
Documentation on how to create theme packs available here.
Builds available on OpenRepos.
I chose this approach as no proper documentation on theming support in Sailfish OS has been found yet.
However, a rudimentary theme support may have been already built-in, since I found sailfish-default folders to have an index.theme package and dconf to have values for those themes. Further investigation is needed.