When your code is split across multiple CSS files, you need control over the order in which your stylesheets are included in the page. In Bootstrap Studio, this is straightforward to do.
You just need to right click on the Styles group in the Design panel and select Include Order.
You can reorder the files as you need them and then click the Save button. These changes will be applied to all pages in your design.
Note that although it isn't shown here, the bootstrap.css file is always included first in the page, before your stylesheets.