How can I change the global font size?

[Lilly (Lillian Louis)]

if you have never done this sort of thing, i would recommend familarize yourself with the basics of working with themes.

Beginner's guide to using Discourse Themes

This is a crash course in Discourse theme basics. The target audience is everyone who is not familiar with Discourse themes. If you’ve already used Discourse theme / theme components, this guide is probably not something you need to read. What are themes and theme components? A theme or theme component is a set of files packaged together designed to either modify Discourse visually or to add new features. Let’s start with themes. Themes In general, themes are not supposed to be compatible …