Brings tabs closer to 8shapes recommendations

+ Lightens the detail header background
+ Inverts the color of header text
+ Shrinks the border between the tab header and content

Before:
![tabs_before](https://cloud.githubusercontent.com/assets/186715/4728934/f839b4da-597c-11e4-8bd1-bf7f464da59d.png)

After:
![tabs_after](https://cloud.githubusercontent.com/assets/186715/4728936/fb9a5cd8-597c-11e4-815f-e2f6112d63cf.png)