Macbook notification banner smudge
Try to completely disable Notification Center, reboot, then reenable.

To do so, (with caution, of course) try the following:


Launch the Terminal app from /Applications/Utilities, enter the following command, and press Return:


launchctl unload -w /System/Library/LaunchAgents/com.apple.notificationcenterui.plist


Reboot
Now re-enable Notification center:


launchctl load -w /System/Library/LaunchAgents/com.apple.notificationcenterui.plist


I have searched in vain to find any information regarding a .plist or other file containing appearance settings for notifications thinking that somehow these had become corrupt.