Trying Shell Lite version 1.5.4 (the zipped package was still in my hard disk, since I considered using this theme at an earlier point!)...
...And nothing yet. :( The columns created by the plugin build themselves, do not get narrower at smaller screen widths (as one would expect on a non-fluid theme), and do not stack either.
Maybe I should be asking if someone who's installed this plugin ever got the columns to stack at narrow screen widths.