If I read that correctly, you’re saying that on the homepage the CSS styles are applied. But then when you click a link (and leave the homepage), the CSS file isn’t loaded?
You can try to use the ‘Network’ tab of your browser’s developer tools to see if there’s an error with loading the CSS asset. You can also inspect the HTML that Hugo rendered when you press Ctrl + U in the browser.
Have you looked at requesting help? That topic has some good suggestions that make it easier for us to help you. 