How to increase the width of my blog (on desktop)

Is there a way to increase the width of my blog (all of it), by making the lines of text longer and keeping font size the same, so that on desktop everything’s not jammed into a narrow column with wide empty margins on both siades? In other words, decrease the margins. Is there a plug-in that can do this? Or is there a simple copy-and-paste change I can make to the stylesheet or something?

So for example:

On my computer this forum we are in right now shows up with the posts 80 characters wide, plus sidebar on the right, and a couple inches of empty space on either side. Using this as a basis of comparison I want my blog text to be 140 characters wide or so (perhaps more), thumbnail galleries the same width, and sidebars to be in addition to this.

Theme is Twenty Seventeen.