You can place this code within a Hook element’s code area. You then set the “Hook” dropdown to wp_head.
On the hook element’s display rule tab, set it’s “Location” to “Entire site”. Save the element.
Upon saving, the code should already be running on the site.
You then go to Appearance > customize > Typography and click the “Add font” button. You can type in “Stockholm Type” on the field like this – https://share.getcloudapp.com/kpu6nB4j – if you wish to use it within the customizer typography options.
A wise man once said:
"Have you cleared your cache?"