Importing externally created CSS in Bootstrap Studio can be done by dragging and dropping it over the app, by chosing File > Import, or from the Design panel, by right clicking the Styles group.
Bootstrap Studio expects the CSS files you import to be valid. If one of the CSS files you are importing contains invalid or incomplete code, you will see an error message like the following:
To work around this, you will need to fix your stylesheet. Try running it through this css validator. It will point you to places in your file that you need to fix in a text editor. After these checks pass, you will be able to import your stylesheets in the app without issues.