css - fixed header footer, 竖直居中，铺满全屏... 老生常谈

参考：  （竖直居中: ） https://stackoverflow.com/a/22218694, 

 底部固定：https://css-tricks.com/snippets/css/fixed-footer/

铺满全屏： height: 100vh https://stackoverflow.com/questions/1575141/how-to-make-a-div-100-height-of-the-browser-window