Improve Notification component colors definitions

[WiXSL]

This change matches info, error and warning CSS rules with material-ui's for the Notification component.

[fzaninotto]

You've already proposed a similar change in #5710, and I said then that we don't want it in core. Why should we merge it now?

Also, this can be achieved in userland through themes.

[WiXSL]

Ok.
This PR was after #6189 was merged but before you revert it, plus I thought the main problem of this type of change was the color of the undo button text which was fixed (before reverting) in #6189. So I move on with this proposal.
Now I understand.
Closing