Hey, this TC doesn’t change doesn’t change the content area or the post width.
What you’re seeing is core’s default post layout.
The post body width is defined mainly by the CSS variable --topic-body-width (usually 690px), next to it, the timeline (min width of 6em) and the main content capped to --d-max-width (usually 1110px).
You can try adjusting those CSS variables. How to increase width of topic - #7 by awesomerobot