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
Swaps tab colors around to match the selection colors used elsewhere
Uses the tab header background color as a base, builds on that.
Bumps header font size for detail pane, differentiates from body text.
Merge pull request #812 from sonatype/8shapes_tabs

Brings tabs closer to 8shapes recommendations