[Disclaimer: I'm not a developer of any sort, but willing to take the time. (Can do a bit of CSS, though.) Also, new to this forum.]

I’m using Anders Norén’s Hemingway theme.

I haven’t tried creating child themes yet and I’m not sure whether a solution to this requires one.

My goal, in short, is to make the whole “Edit Post” webpage look like an offline word processor:

1. Make the Visual editor occupy the whole screen. (I use the classic editor.) (Done: Collapsed the vertical menu on the left side, selected 1-column layout, and disabled “full height editor and distraction-free functionality.”)
2. Set a width for the text (because currently text is almost from edge to edge of screen)
3. Put text at the center (but “aligned left,” not “justified”). This is for the "left and right margins," within the post editor.

It now looks like this.

How do I do #2 and #3?

Thank you.

Edit: Just to clarify: I'm looking to change how the Visual editor looks--only the Visual editor, not any output the live website would show.