Currently for some reason it's not possible to changezIndex for Toaster component. But it's very likely to be possible.
The documentation says that this component has a property calledzIndex, but as I said it's impossible to use it:

One of possible solutions:

importToasterfrom'evergreen-ui/src/toaster/src'consttoaster=newToaster({zIndex:40})toaster.danger('Error occurred!')

Another solution:

import{toaster}from'evergreen-ui'toaster.danger('Error occurred',{zIndex:40})