NIKLAS BRUNBERG  I see. This is a Web Kit style (little more than CSS, XHTML and graphics) and the colors for the priorities are set as graphics. To be able to control additional parameters I would have to redo this as a Cocoa (Objective-C) style.

I have been thinking about converting this to a Cocoa style because it would result in more control for users such as you and it would also mean a lower memory usage (because some issue with Growl and Web Kit styles leak a bit of memory). However, those plans are not close.  
(Version 1.1)

NIKLAS BRUNBERG  You have to set them on a per application basis. Go to the Applications pane, double-click any application you wish to have a special priority, select the Messages pane and you can set the priority for notifications from that application.  
(Version 1.1)

NIKLAS BRUNBERG  Yes, I have seen that happen once in a while, looks like the web kit engine did not load the css file. I have no real idea on how to fix it. Try updating Safari (if you have not already) and install a fresh copy of the style. You might also try logging out and in again (or restarting your computer) to reload the web kit in Mac OS X:s memory.

Please tell me if you still suffer from this problem or if you managed to solve it.  
(Version 1.2.2)

NIKLAS BRUNBERG  I have made the colors more neutral for those priorities. Not sure about making the font in the title bar bold. I want to hold it true to the Apple style and if you look at the HUS:s in iPhoto and Safari you will see that they are normal weight.  
(Version 1.1)

NIKLAS BRUNBERG  This is a modified version of the bundled display style NOtifyOSX.   
(Version 1.2.2)

NIKLAS BRUNBERG  Yes, it might be a problem with the plist file. Try removing com.growl.growlhelperapp.plist and installing the style again.  
(Version 1.2.2)