Nicely done! What's going on here?
The order of the CSS files matter.
1. The browser first sees shift.css, and makes the Shift font available to use in the page
2. Next the browser sees bootstrap.css, and applies those styles to the page.
3. Then the browser sees main.css, and applies those custom styles to the page. The custom styles override the Bootstrap styles.
The order of the CSS files tell the browser that your styles in main.css are more important than the styles in bootstrap.css, so the final page that loads is the customized web page.