Applying Style Kits to pages from the WordPress dashboard
Indicator for Pages that do not use the Global Style Kit
This only takes place if you have a Global Style Kit activated. If you do so, in your pages you will see a blue dot that indicates that the pages that use different Style Kits than the Global one. This way, you have a quick view of the pages that do not follow your Global Styles.
Apply the Global Style Kit
On any of those pages with the blue indicator, you will find an action to Apply the Global Style Kit to them.
Apply Style Kits with Quick Edit
Quick edit mode has a Style Kit drop down so you can quickly assign a Style Kit to any page, without opening it at the Elementor editor. Choose the Style Kit that you want to apply and hit update.
Bulk - applying a Style Kit to multiple pages
You can also macro-manage Style Kits through Bulk Edit. Simply select multiple pages by checking the boxes, and click on the top-left dropdown.
Hit apply and you should be able to edit all your pages in Bulk. Notice the Style Kit dropdown at the bottom left. It's where you can select the Style Kit that you want to be applied on those pages.