Incorporating CSS into your C# code in ASP.Net is a straightforward process, regardless of whether you are using ASP.Net MVC, WebForms, or raw ASP.Net, as CSS is essentially just a file with a specific content type.
If you only require a few overrides, one approach is to include the CSS directly within the page.
In ASP.Net MVC, creating a separate CSS file involves generating the text in the controller and then returning it using the Content method. For more details on this process, refer to Display contents of Text File in MVC3 Razor. Alternatively, you can create a view that renders only CSS without any HTML tags, utilizing Razor constructs. Remember to set up a route and consider adding <modules runAllManagedModulesForAllRequests="true" /> Meaning if you plan to use the ".css" file extension.
Keep in mind that depending on your specific requirements, using SASS or LESS for CSS preprocessing may be a better option. While these tools do not support dynamic "colors for current user" scenarios, they allow you to generate consistent and easily modifiable CSS (e.g., ensuring that the button color remains uniform and can be updated across all CSS files simultaneously).