The default width of Twenty Seventeen is about half the screen. That was unsatisfactory to the editors who installed the Theme. WordPress Happiness Engineers provided two snippets of CSS code which allow us to change the width of the content area.
In the event this code is changed or lost, this is what we started with:
/Change width wrap homepage | ZD-3414659 | WH/ @media screen and (min-width: 30em) { .page-one-column .panel-content .wrap { max-width: 1400px; } } /* Change width of non-homepage pages / 3414659-zen NS */ @media screen and (min-width: 30em) { .single-post:not(.has-sidebar) #primary, .page.page-one-column:not(.twentyseventeen-front-page) #primary, .archive.page-one-column:not(.has-sidebar) .page-header, .archive.page-one-column:not(.has-sidebar) #primary { max-width: 1400px; } }
The last entry in each block of code is the max-width parameter, in pixels.
Reducing from 1400 is fine; increasing may produce unpredictable results.
Editing the CSS (Cascading Style Sheet) Code

To edit the CSS code, select
My Site — Design — Customize — Additional CSS
Be careful what you change.
Only the max-width numbers should be changed;
and only down from 1400.