I’ve created a Growl style before. Now I’ve created one again. I call this one SimpleGradient, because that’s what it is: a simple gradient from #ddd to #bbb. It also has rounded corners, and the text seems sunken into the background (like the menu and post info blocks on this site). Other than the fact that — save for the application icon — it uses no images, that’s basically it.
Anyway, it looks like this:
SimpleGradient Growl style
If you like the minimalistic design and would like to use it yourself, then go ahead and download it.
Be sure to check out my other Growl style.
I’ve been messing around a bit with Growl styles. Those are just some HTML, CSS and images in a package, so it isn’t all that difficult to make your own.
All this playing around resulted in an all new Growl style. It’s inspired by the semi-transparent toolboxes found in some OS X software (iLife for instance), therefore I named it ToolBox. And this is what it looks like:
ToolBox Growl style
So you want it too, eh? Well, you’re in luck, you can download it right here! 
Download
Oh, and it works best with opacity set around 90%.
Those vague vertical lines in the screenshot are probably an artifact from OS X making the screenshot. They are not there in reality. The images used for the background are fully opaque (except for the corners) and are made transparent by using the “opacity” CSS property with a value spplied by Growl (and ultimately you through the preference pane).
Be sure to check out my other Growl style.
