NotifyOSD notifications have looked the same since they were first introduced with Ubuntu 9.04 Jaunty Jackalope. But NotifyOSD will soon get a new look: the notifications will use the same median colour and blur used for Dash (which depends on the colour of your wallpaper).
This change is already available in the latest NotifyOSD 0.9.33, released today, but it requires Unity 5.4.0 which hasn't been released yet.
Here are some screenshots by Mirco Müller:
Unfortunately, there's no mention if the colour will be configurable, but since Dash can be recoloured, NotifyOSD will probably support this too. And if it doesn't, the unofficial patch that lets you customize NotifyOSD will hopefully be updated so you can change more than just the color.
What do you think?