- Download All Pages Zoom from this website.
- Go to Preferences... from the Safari menu on the Menu Bar. (The second item under the Safari menu is Safari Extensions... While it makes sense to choose this menu item, it takes you to Apple's Safari Extensions website.)
- Choose the Extensions view.
- Choose AllPagesZoom from the left pane.
- Set the Zoom level. I suggest starting at 115% and adjusting up and down from there are needed.
All Pages Zoom is free, but the developer accepts donations for his work.