Most of Web Browsers Support F12 Option for Developers to customize CSS but just for checking website Viewport but you can Customize any Website Style Permanently for you and your Browser by Help of Some Extensions:
Mozilla Firefox Extension -
Smart and Clean but needs little CSS knowledge, but you can give a chance to Google some CSS properties.
Google Chrome Extension -
Really Simple to Use Chrome Stylebot, as it can work for new users by customizing simple available CSS Properties with Drop-Downs and Input Fields.
and there are many more to view for your custom Browser, just Google it and yes for now Apple Safari and Windows IE not support these Extensions.