The amount of space dedicated to actual posts has not changed, so I don’t really understand the issue. The max-width of those is the same, the only thing that has changed is the sidebar not being empty as it was before… I’ll have a look at the dark mode..
PS. The plan is to add in a show/hide for the sidebar, but I’m still playing around with that… I also plan to add other features there… Hiding the sidebar won’t provide any additional room for the posts though since those don’t get wider and never have.
PPS. If your sidebar is actually taking up more than about 15% of your page width, then it could actually be a rendering issue on your browser… Send a screenshot if that’s the case! I fixed the issue reported by @Zebras though, so no one should be seeing the sidebar taking up half their screen anymore.