The parameter you're looking for is Text-unread and you can set it any way you like. Anything you don't specify in the skin's skin.properties file gets filled in by the default values specified in the _base/base2/skin.properties file.
The default value is:
and the @Text@ value stands for color: @TxtC@; where TxtC is #000000. So, to achieve what you want you should add something like the following line to your skin:
Text-unread = @Text@ font-weight:bold;
... I haven't tried it, but I think you can also change it in the base2 skin and it should apply for all (unless they overwrite this value).
Text-unread = color:red; font-weight:bold;