Updated style to use CSS3 features + clean structure

Style has always been a mess in Chive. I tried to clean up the whole CSS code and to utilize some of the new CSS3 features like gradients instead of images. Only small changes/fixes have been made to the style itself.

![Screenshot from 2013-04-21 15:18:59](https://f.cloud.github.com/assets/229567/406356/1957011a-aa86-11e2-95c7-fc0bf22b41de.png)

**What has been done?**
- Use CSS3 gradients instead of images
- "Separation of concerns" for layout and style
- Cleaned up file structure
- Cleaned up CSS rules

**Tested in:**
- [x] Chromium (Latest, Linux)
- [ ] Chrome (Latest, Windows)
- [ ] Firefox (Latest, Windows)
- [ ] Internet Explorer 8
- [ ] Internet Explorer 9
- [ ] Internet Explorer 10