Fontain is a simple Ace3 addon to give you a highly configurable set of controls over your UI's font choices.
You can set a master font for your whole UI, and even override SharedMedia so that all addons that use SharedMedia will use your selected font. Additionally, you have fine-grained control over every font in your UI, with a powerful wildcard matching system.
For example, if you want to set your master font to "Calibri", but want your tooltips all in "Bazooka", you could set the font for GameTooltip as you see fit, and all font strings that match GameTooltip* will be updated, including all derivatives. While this is often already done via inheritance, it does catch a number of outlying cases, giving you very powerful control over your UI's font selections.