We use Google Fonts in our theme, that make it easy for you to change:
- Go to Google Fonts and find the name of the font you want to use: http://www.google.com/fonts
- Copy the name of the font
- Go to Tumblr, click "Customise" on the right
- Paste the name of the font you want to use into Logo/Heading/Body Google Web Font to change as needed
Colours can also be changed by altering the settings in the "Customise" panel. Unfortunately you can't change the size of the text in your theme, as this has been carefully chosen to look legible and correct as part of the theme design.