Hey Rich... long time! How are things over at OGR?
Below is the pertinent custom CSS. I post it with caveats:
Caveat 1: We aren't using the "cards" mode/version of Cstack. Like you, we're using what I believe used to be called "version 1" or the legacy format, (i.e. no cards enabled).
Caveat 2: We're not running any third-party features (code). If someone is, then our width adjustment may create unintended (but not necessarily unfixable) changes.
Caveat 3: Over the years, we've created quite a few lines of custom CSS to change the appearance of Cstack's interface. After implementing the "80%" change, we had to tweak the layout of a few of our items (but not a lot). From time to time, Cstack has looked at/helped with our customizations and given us advice. However, Cstack Support has twice told me they don't support changing the width of our site and won't help us do so. I'm mystified by that, especially since, SO FAR, we've proven that the change is easy to do and seems to work without negative consequences.
Caveat 3: I'm not an expert in CSS. I work at it until it works. I'm sure a pro could clean this up and maybe do it in a different way -- but IT WORKS for us.
Thankfully, none of the following affects responsive "small screen" appearance.
Good luck.
Neil
/*this one took us from 66% to 80rem*/
.row {margin: 0 auto; max-width: 80rem; width: 100%;}
/*increase community nav bar width for wider screen*/
#top_nav_bar_wrapper {max-width: 80rem;}
/*adjusts sticky community nav bar titling to new width*/
.row .fixed .row { max-width: 80rem;0}
/*needed to adjust nav bar to new screenwidth when in stickyfixed position*/
.fixed {right: 0;}
/*seems to help top nav bar stay inside its container. Original maxwidth was orig 62*/
.contain-to-grid .top-bar {max-width: 80rem;}