When you view a webpage, Safari uses the Cascading Style Sheet (CSS) you specify in addition to any style sheets specified by the page. A style sheet specified by the webpage may override your style sheet.
Open Safari if it is not already open.
Choose Safari > Preferences, and then click Advanced.
Check the “Style sheet” pop-up menu to see what style sheet is selected. For more information about the options, click the Help button (looks like a question mark).