I use firebug (or you can use chrome web developer tools) to inspect the element, in this case the thumbnail. This brings up a panel at the bottom of the screen where you can select an item in DOM and see the applied css styles on the right, edit them to get the effect you want, then cut and paste the code back into your theme.

Videocast: http://screencast.com/t/afDfdzOdaFaj

http://getfirebug.com/