How to change Bootstrap's global default font size?

There are several ways but since you are using just the CSS version and not the SASS or LESS versions, your best bet to use Bootstraps own customization tool:

http://getbootstrap.com/customize/

Customize whatever you want on this page and then you can download a custom build with your own font sizes and anything else you want to change.

Altering the CSS file directly (or simply adding new CSS styles that override the Bootstrap CSS) is not recommended because other Bootstrap styles' values are derived from the base font size. For example:

https://github.com/twbs/bootstrap-sass/blob/master/assets/stylesheets/bootstrap/_variables.scss#L52

You can see that the base font size is used to calculate the sizes of the h1, h2, h3 etc. If you just changed the font size in the CSS (or added your own overriding font-size) all the other values that used the font size in calculations would no longer be proportionally accurate according to Bootstrap's design.

As I said, your best bet is to just use their own Customize tool. That is exactly what it's for.

If you are using SASS or LESS, you would change the font size in the variables file before compiling.

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.

You can add a style.css, import this file after the bootstrap.css to override this code.

For example:

/* bootstrap.css */
* {
   font-size: 14px;
   line-height: 1.428;
}

/* style.css */
* {
   font-size: 16px;
   line-height: 2;
}

Don't change bootstrap.css directly for better maintenance of code.