Replace background gradient images with CSS gradients

[nipos]

The website still uses background images for gradients in some places rather than CSS gradients, which are supported by all browsers nowadays. That can lead to non-optimal quality especially on high resolutions or when zooming in, since images don't scale losslessly while CSS gradients do. nephele asked me in the chat if I could change the page background to use CSS gradients, so I did, and also for the background of the icons on the top. I'm attaching the modified shijin4.css file here to make sure that whole topic doesn't get lost in chat history.