Hi Mark,
in the themes\whatever folder is nettalk-ui.css
inside this are the 4 lines that govern the browse colors;
.nt-browse-gb1{background-color: #FFFFFF;}
.nt-browse-gb2{background-color: #F8F8F8;}
.nt-browse-mouseover{background-color: #DDDDDD;}
.nt-browse-selected{background-color: #CCCCCC;}
(the above is from the shoestrap theme, but obviously the colors will vary from one theme to the next.)
Remember you don't need to edit the nettalk-ui.css file - if you add any of the above lines to your custom theme file then that will override the nettalk one.
cheers
Bruce