Hellmark, The way to change fonts is with the calibre plugin that is used to create/manage Kindle collections. When your Kindle is plugged in (from the Home Screen), create a folder called "fonts", and plop the fonts you wish to use into the folder. Then go to Manage Kindle in the dropdown menu of calibre's plugin.
If it doesn't like the filenames of your fonts (it didn't like the filenames of mine even though they were all the same set), change them to what it wants, then go into the manage and click "enable user fonts" and select the fonts of your choice.
Eject your Kindle, unplug it, then select "settings" on your unplugged kindle, and then select "restart" for the configuration to "take". When it comes back up the font you chose should be selectable for an open book.
Hope this helps.