Updated design of PreviewWidget simmilar to Opera 12

Preview Before:
![previewwidget_before](https://cloud.githubusercontent.com/assets/720808/4916588/bcf3a494-64da-11e4-89f4-afa0b32e3bf4.png)
Preview After:
![previewwidget_after](https://cloud.githubusercontent.com/assets/720808/4916589/bebd570c-64da-11e4-8686-0c65a0f9dbbd.png)

It was annoying for me to see that yellow preview. I've added some CSS styles.
If you are planing to have Skins in Otter than these styles should be moved to a skin.