Here you can select a font type and colour to use on your navigation menu, all footers (mailing list subscribe, footer, date/time etc)... ie all text on your website apart from the page content. 

NB: The reason that you can't find the fonts from your hard drive in the menu and that there might not seem to be a limited choice of fonts is that only a certain set of fonts will work consistently across the internet and across all of the various clients that will be accessing your website. The fonts listed in the menu are the current "web safe", fonts.

Step 1:
Click on the 'design' tab, select the 'customise theme' option and then select the 'Font Style & Colour' button.

Step 2:
Under the Font Family menu select the font you wish to use on your website.

Step 3:
Select a font colour by either typing the hex number of the colour you want into the Font Colour field or if you don't know the hex colour just press the Colour Picker button and you'll be able to choose a colour from the colour picker menu.

Step 4:
Press the Save button to save your changes.