@import statement that you copied from https://fonts.google.com needs to be at the top, and that would not be the case if you add it on the Global CSS. You can add it on the Additional CSS, or on the page Custom CSS level though.
However, having a child theme now might save you time in the future.
Your code given above can be place on Global CSS, if it does not work please update that to this:
font-family: "aktiv-grotesk", sans-serif !important;
Hope it helps,