When it comes to URLs, there is no distinction between a URL specified in a
<link rel="stylesheet" url=..
tag and a URL entered by the user in the browser address bar.
Both serve as endpoints that the browser needs to access. It is possible to secure your entire application behind an authenticated endpoint, but once the client gains access and the page is loaded, users can still view your styles using developer tools.
One potential workaround is utilizing tools to minimize and obfuscate your styles. While this can aid in minification and optimization efforts, it may not necessarily safeguard your sources. Consider what exactly you are trying to protect (CSS styles themselves are typically not confidential information, and copyrighting your logo/brand style may offer more comprehensive protection).