Actually, for anyone interested, I have just discovered how to change font size. You can do it with the themes in %config.js.
Every theme in %config.js has a base section. You need to add a new setting to it. Here's what mine looks like now:
"base": {
"background-color": "#214c80",
"color": "#e6f0fc",
"font-size": "18pt"
},
This gives me a nice comfortable reading size on my laptop. The "pt" means that the size is specified in points, but since this seems to work using web styles, you can probably also set pixels ("px") and so forth. This also means you can probably style other things about the text using web styles (CSS) too...
Note that if you want to have the same font size in all themes, you will have to copy that line to all of them.
Hope this helps!