Give random image page an imgur colour scheme

* Sorted and formatted CSS
* Pushed "Loading..." callout to left
* Gave callout a curved edge
* Coloured the background black
* Coloured the callout text green
* Coloured the callout background grey
* Changed callout font to sans-serif
* Increased callout padding
Merge pull request #366 from zuzak/patch-5

Give random image page an imgur colour scheme