Subtheme system: JS folder & bootstrap.css not loading any more

This was picked up by @ufundo yesterday - that the /js/ directory isn't loading any more, which is perhaps connected with the change to sub-themes here: https://lab.civicrm.org/extensions/riverlea/-/blob/766de66bd80288844f194b201c60236fd7e98622/riverlea.php. Already that change stopped Bootstrap.css from loading. I tried a range of options and locations for the file but it wouldn't load - and so fudged it here: cf47b685.