Create a web page design that has a header, a sidebar on the left, a main content area to the right of the sidebar and a footer below the main content area. The website width should be 960 px. The left sidebar should be 300 px wide with a 20px left margin. The header should be 100px high and 100% wide. The footer should be 60px high and 100% high. The right content area Bootstrap and flexbox should be used in the design css.