update border colors in tooltips after recent color scheme tweaks

This will be obsolete as soon as *all* tooltips are native SVG, but until then update the tooltip image border colors to match the recently tweaked color scheme (see card https://trello.com/c/a7MS8wBn).

Card for this task: https://trello.com/c/4MalzfVy

Before (screenshot from viz test page):
![screenshot 2014-07-23 19 09 19](https://cloud.githubusercontent.com/assets/1588547/3682890/322ce266-12d8-11e4-8cbc-2f38aeb65e71.png)

After:
![screenshot 2014-07-23 19 08 39](https://cloud.githubusercontent.com/assets/1588547/3682902/3d99d7da-12d8-11e4-857a-8111b88993d4.png)