Thanks Jeff...that one is specific to WebHelp, whereas I'm looking to edit CSS for Responsive HTML. I watched Willam van Weelden's webinar on the topic, which is for RH11.
RH2015 seems to have additional elements that need to be modified, as modifying only the sidebar width and main topic pane width doesn't do the trick. The show/hide sidebar icon doesn't shift with the sidebar itself, for example. It's not a 1 to 1 match, so I was wondering if anyone has actually done this for RH2015 and if so, exactly which elements get changed?
The best way to do this is through the SCSS files and compile the CSS file from that. There are many different elements that use the sidebar width for correct positioning.
In the SCSS files, you find the variables:
By updating these in the SCSS, the sidebar will be shown correctly. You can get the SCSS files (and instructions) here: https://helpx.adobe.com/robohelp/robohelp-screen-profiles-layouts.html
The download link on that page to the SCSS 2017 zip files (scss-files-2017.zip) is visible but the one for 2015 (scss-files.zip) is hidden. I had to view the page source code to find the link.