Increase right margin of text

Can anyone tell me how to change the width of the body text in posts (line width, not width of each letter)? I asked in the forum for the theme I’m using, Twenty Seventeen (11 days ago) and didn’t get an answer. Specifically, I want to increase the right margin while keeping the left margin as it is. The width and placement of everything else such as headings, images etc. should remain unchanged.

If you need to see the site, please let me know some way I can provide the URL privately because posting it here in the forum isn’t an option.