You have to set the tone for every each of them. Settings > Notifications & status bar > App notifications. There you can set notifications and the tones for the single apps
For the people, complaining about the icons: You can use the MultiROM icon pack, it's not exactly the same, but it has squared icons.
Download MR-Icons_V9_8.7.4.mtz
seems like it's kind of an indicator on how symmetrical your face is... so pretty useless I guess
Edit: after a little reseach: It shows, how 'pretty' you are on that selfie... so even more useless
I guess, it will come later, since official MIUI Rom for Mi6X now seems to have Anti-Rollback too, and the developers have to fix this in the Xiaomi.eu-Rom