If you have a stylesheet that you wish to quickly import in the program, without having to write the CSS by hand, you can do it in one of two ways. The first is to right click on the Styles group in the Design panel and select Import CSS File. The second - just drag and drop your CSS file in the app.
Bootstrap Studio requires 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.