Brickset API documentation is available here: https://brickset.com/article/52664/api-version-3-documentation/
getSubthemes(theme, key = getOption("brickset_key"), ...)a data.frame with the subthemes.
the theme.
the Brickset API key.
other parameters passed to getUserHash including
the Brickset username and password if they are not available from
getOption('brickset_username') and getOption('brickset_password').