Tumblr themes: Make your font sizes bigger or smaller
We are afraid that's not possible. As all of our sites are responsive we very carefully plan how the text is going to render and read on lots of different devices, from phones up to huge monitors. This means that we feel that it's better to keep the text sizes as they have been set up for you.
If you are familiar with CSS it probably wouldn't be too difficult to work it out but it's not something we can really help with at this point.