content banner - styleguide

We use [gulp-autoprefixer](https://github.com/sindresorhus/gulp-autoprefixer), so we don't need to specify all browser-specific CSS properties. You just need this line:

```css
background: radial-gradient(ellipse at center,  rgba(233,229,224,1) 0%,rgba(255,255,255,0.45) 55%,rgba(255,255,255,0) 100%);
```