Jak - You're Welcome, however k-putt did make a theme that was less eye straining long before me but it was more light gray than anything else.
Notato - You'll have to resort to Stylish for that. Here are two example images of how you can set it up.
First image- This is site wide and effects ALL text areas, however I'd only use this if you just wanted one font for the whole site which I only recommend if you want just the font changed: https://i.gyazo.com/48b23bf680d72fc7c388794e52ee79c8.png
Second Image - This is more specific text areas, with this you'd be able to change size and/or change the font without the whole site being changed but specific portions of it. I recommend this is if you would like to mix and match font families or sizes: https://i.gyazo.com/4b78423a0d8c5fa8e42f6d9e4bc1788c.png
A couple of things to note:
First, I have no idea how many font families there are that you can use but I'm sure a Google search will help you there.
Second, if you want to change the size of the fonts the line would be "font-size: 12px;" without quotations inside the brackets, it can be below or above the font-family value and they can be larger or smaller than 12px.