Making a Global Style Kit
A global Style Kit is a Style Kit that applies for all the pages at your site. If there is a page that has a previously assigned style Kit, that style Kit will remain assigned to it.
The Global style kit applies for all New Pages / templates or any earlier ones that do not have a Stylekit manually applied to them.
At the Editor, Under the Style Kit tab, you will find a text link Under the Save As button that prompts you to Set a Global Stylekit.
Choosing the Global Style Kit
Clicking on the text link above, will take you to the Elementor settings page, and specifically the Styles Tab. There, you can Set a Global Style Kit at the Dropdown.
Now, on every page the Global Style Kit should have already be already been applied. On the pages where the Global Style Kit is in effect, you get a notification that you are editing the styles of the Global Style Kit.
Indeed, if you make changes to this Style Kit and hit the Update Style Kit button, you just made a site-wide change.
You can always override the global Style kit at any page, by simply choosing another Style Kit from the dropdown.