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Due to declining eyesight, monospaced fonts are way easier for me to read. Is there a way to switch the default font High Sierra uses out for one that's monospaced? If it's not supported natively, are there any 3rd party programs that can?

I think you can change the system font, but I don't know if that's a good idea. The OS may react in "odd" ways if, for example, a character it displays is missing from the font you've chosen. It might work perfectly - or it might not... all I'm saying is that I personally have no experience with it. And having no experience, I wouldn't do it unless I was dead-certain I had a way to restore the default. Other answers to your question may have this covered, so please feel free to ignore my paranoia :)

All of that said, I actually do have an answer for you, but it uses the existing system font - just in a larger font size. Making that change is easy, and it's easy to remove the change if you don't like it, or want to try something else:

1. Start (or click on) System Preferences, select the Displays tool, and select the Display tab.




2. Next to the Resolution option, select the Scaled radio button. Your dialog box should now look like this (at least on High Sierra 10.13.6):

3. Select one of the optional larger fonts; you can try different sizes until you find the one that works best for you.

Hope that helps. If you're committed to trying a monospaced font, there are numerous videos and blogs that propose approaches for doing that. Google the phrase mac os change system font to get a large selection. And finally, here are some tips from Apple to make your screen easier to read.

Unfortunately there is no built-in way to change the system default font, there are, however, a couple of utilities that let you change the default system font or at least some of the system fonts. How well this will work with a monospaced font like Courier is anyone's guess, and mine would be probably not very well.

You can change the font back to something many thought was more readable than the current font. By changing it to Lucida Grande.

There is another utility called TinkerTool that lets you change the font for some of the macOS. I don't believe it will change everything but it likely worth the try.

Those are all I know of, there may be others that work better but these should give you a good start.