This is a Flat Red theme for the app Floating Notifications.
You can grab Floating Notifications free trial on the Play Store here:
You can find many more themes for Floating Notifications on XDA here:
I made it to match the Simplex theme chooser theme here: https://play.google.com/store/apps/details?id=com.mac.theme.Simplex&hl=en
Enjoy and let me know what you think!! Feel free to email me any time.