How do I change the menubar transparency in Lion?
You would most likely need to use a translucent/transparent background image to accomplish what you're asking. If you know how to make an image 90% transparent then you should be able to experiment and to come up with something similar. Tweaking the UI is art in many ways, however, it's not necessarily a function of the interface creating the effect; the elements that compose it (background color, wallpaper, dock and icons) play a key role. 

[posted for reference] 

System Preferences > Desktop &amp; Screen Saver > Translucent menu bar




One app that I've used to achieve a similar effect is called Nocturne. I'm unsure of it's compatibility with Lion though.