I’ve made a very simple Theme Component which widens the Topic on Desktop only. It was made for my personal notebooking/blogging Discourse at bawmedical.co.uk and it’s not very advanced and doesn’t use @media queries (yet). But it may suit simple use-cases. If anyone uses it and has feedback then we can create a new Topic for discussion of it here on Meta, or submit Issues or PRs on GitHub
https://github.com/pacharanero/discourse-topic-width-desktop
(This is as much an aide-memoire for myself as anything else, because in $sometimeperiod I will have forgotten how I did this and will be searching Meta for it again!)