When using multiple CSS files, you often need control over the order in which your stylesheets are included in the page, so that styles are not overriden. 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 drag and drop the files in the order you like 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.