Brilliant Directories' default CSS stylesheet can be deactivated using the Advanced Setting: website_disable_css_stylesheets

In order to activate this Advance Setting navigate to Setting - Advanced Settings:

Once the Advanced Setting has been turned on and since there will be no link to a stylesheet, the website will look broken like this:

If the custom CSS stylesheets are hosted elsewhere, the way to link this will be using the Custom CSS / HEAD section,

Additionally, custom CSS can also be added directly to each static page. To do this, navigate to the webpage that needs to be edited and use the CSS tab: