Currently the toggle for "limited content width" / "wide content width" is an easter egg: a small icon at the bottom-right of the screen that is easy to miss even if you're looking for it, and only appears sometimes [at certain window widths/resolutions].
* Make the icon more visible: larger, an extra 30px away from the R and B edges, and with a more defined boundary or color/shade contrast
* Make the hovertext more understandable, and make it appear more promptly on hover. Currently the text always says "toggle limited content width" which doesn't mean much to readers. This could read "make page content wider / narrower", changing with the current state of the page.
* Make the icon more consistently there: for a broader range of window widths. T326887 is a start, but will still be wider than I or the other commenters on that ticket expected. If I can see that the body text isn't flus with the right side of the topnav, I'd like the icon to still appear.
* Until there's a cookie or other way to persist this toggle for logged-out users, please also show they a message (only the first time per session, or the first time per IP?) inviting them to create an account and log in to keep that setting throughout their session (to avoid confusion when on the next page load it changes back).