How to change Bootstrap’s global default font size?

Bootstrap uses the variable:

$font-size-base: 1rem; // Assumes the browser default, typically 16px

I don’t recommend mucking with this, but you can. Best practice is to override the browser default base font size with:

html {
  font-size: 14px;
}

Bootstrap will then take that value and use it via rems to set values for all kinds of things.