factorise the demo css code; also avoid using linear gradient

some browsers are slow to render it the linear gradient

The data url is built with the following code

```
var canvas = document.createElement('canvas')
canvas.setAttribute( 'width' , 100 )
canvas.setAttribute( 'height' , 100 )

var ctx = canvas.getContext('2d')


ctx.strokeStyle = '#eee'
ctx.lineWidth = 0.5

ctx.beginPath()
ctx.rect(20.5,0.5,20,100)
ctx.rect(60.5,0.5,20,100)

ctx.rect(0.5,20.5,100,20)
ctx.rect(0.5,60.5,100,20)

ctx.stroke()

ctx.beginPath()
ctx.lineWidth = 2

ctx.rect(0,0,100,100)

ctx.stroke()


document.getElementsByTagName('body')[0].style.backgroundImage = "url(" + canvas.toDataURL("image/png")+ ")"

```