BuddyPress.org

Thanks @r-a-y. It could be done without a plugin by uploading the desired theme files into wp-content/themes/default folder to replace the files there. Then change the Theme Name of file of new default theme’s style.css to Theme Name: WordPress Default

Just keep a copy of the preferred theme in your computer so that whenever you upgrade your WP install, you upload your preferred theme files with the revised Theme Name in style.css again to wp-content/themes/default. Since upgrading WP occurs around 5 times a year or so, no hard task at all.

The only hitch is if the replacement theme has hardcoded paths instead of using relative paths and therefore would not work in folder name different from referenced files’ URL’s.