Comment by orojackson

For people in the future who want to apply the centered viewport classless version of Pico CSS, just apply the following in Firefox's Style Editor (F12 to open DevTools, then click on Style Editor; click on the + sign to add a new style):

  @import url("https://cdn.jsdelivr.net/npm/@picocss/pico@2/css/pico.classless.min.css");

I honestly did not know that until I took about 5-10 minutes looking up how to apply arbitrary styles in Firefox.