Thanks Ron. I hadn’t spotted the different classes on the <body> tags. That will let me do what I think I need. I understand it looks confusing but I need the full-width page (just the one at the moment) to behave differently from the pages with a sidebar in the way they cope with different size screens and after exhaustive experimenting I’ve identified #container as the key. The original responsiveness.css works very well and only needs a little tweaking for my appplication so thanks for a nice build.
I’ll leave a review once I’ve finished developing the site, but this is my first venture into WordPress and I feel lucky to have hit on Frontier as a theme more-or-less from the outset. More luck than skill! I’m comfortable with HTMl and css and know a moderate amount of Perl, but php is a new experience and I couldn’t work out how to add a class to #container. Your solution’s much better because I don’t have to edit a template file.
Thanks again, and for a prompt reply!