You can change background color of enhanced tooltips, if you are using VS2012, Resharper >= 8.2 && < 8.3 with Enhanced Tooltip plugin v2.2.1. Here is how.
First of all install VS2012 extension "Visual Studio 2012 Color Theme Editor". Restart VS. Go to Tools - Customize Colors. Click on one of installed styles (on hovering there is icon named "Edit Theme"). In the opened tab Scroll down to "Tool Window - Tab". Change background color for "Selected, Active". This is also background color for enhanced tooltips.

P.S. There is new version of Enhanced Tooltip and background color now is taken from Text View...