customizing cognos navigator portlet

[gvmnh]

hello, everyone

need help in customizing cognos ui. we use pretty straightforward approach, when selected report in cognos navigator is shown in cognos viewer portlet. what we want is to highlight selected entry in cognos navigator using bold font or background colour , so users can see what report they have chosen. i suppose we need change some css or xslt files, but dont know which one and where...
tried to make changes in default.css file , didn't help :(

thanks in advance

ps . we use cognos 10.1.1