Consider that you want to change your application Header, with new fonts, colors and an...

Change the Common/Header Web Block, by adding a Container with the new icon. Add the new CSS to the Module Theme's Style Sheet.

Add the new CSS to the Style Sheet of every Web Screens in the application, overriding the Style Sheet of the Common/Header Web Block Replace the Header_Logo with the new icon.

Change the Style Sheet of the Common/Header Web Block, with the new CSS implemented and replace the Header_Logo with the new icon.

Replace the Common/Header Web Block on every Web Screen with a new Web Block containing the new icon and CSS in its Style Sheet.