In mono, skin designs are designed and differenciated by the fonts they use. The easiest way to change the font of a site is therefore to change the skin. If you need to use a font that is not a standard in mono skin designs you can easily use Google fonts. Learn how here.
The size of fonts on a site is also a important visual factor. In mono you can easily use a few CSS snipets to change font sizes in your menu as well as your headlines and basic text modules. You can find all the snippets in the mono CSS Library. Be sure to check all the modules if you make several changes in font sizes in your skin.