This can be done via the paid CSS upgrade using an "opacity" declaration, but it is somewhat tricky because Internet Explorer and older versions of Firefox require special handling, so you end up with needing three lines of code to take care of all browsers.
There is also a trick where you can actually specify a background color using RGB instead of hexadecimal color codes and then specify an alpha channel transparency.
The first solution using "opacity" would make the grey background as well as the white lettering more transparent.
The second using the RGB/alpha channel transparency would only make the grey background more transparent.
The other possibility is to simply do away with the grey background for the tabs and just let the white text float over the image.