Currently, the home page of my website is on full-width, which I’d like to keep.
I’d like to change the with of my individual blog posts pages (but I cannot do this in Customization as the home page changes too).
Basically, I would like to make the content more packed – i.e. increase the margin on both sides to look something like this: https://imgur.com/kgP0Azo
You can create a Layout element with it’s Content Area set to “contained” and Content width set to 1200px.
You then set the Display Rule location to “Posts – all posts” so it only applies on single post pages. Add more to this if you want it to apply on other pages like Category Archives, etc.