Hi Carter,
You are referring to the Design System. I would use that if you have multiple apps that require the same style.
https://docs.mendix.com/howto/front-end/create-a-company-design-system/
If the App is a one-off, you can use the Styling section:
https://academy.mendix.com/link/paths/123/Style-your-App-with-Sass
You can bing add-ons also to other projects using a Partial:
https://academy.mendix.com/link/modules/481/lectures/3864/-4.3.1-Create-a-Partial
And more advanced Styling can be found here:
https://academy.mendix.com/link/paths/122/Polish-your-Styling
Go Make It