Fix 427

[aress31]

This PR is aimed to address ticket #427 by implementing a better integration with the Material UI theme.

Question to @iamhosseindhv, do you prefer using the main or light theme variant for normal/light theme. I went with main as IMHO it is bright enough for a light theme and provides a sufficient contrast ratio.

I also replaced the hardcoded padding values with theme.spacing(n) so that global changes to a theme are directly reflected in the snackbar.

Note: This PR is based off the next branch for support with MUI v5.

[iamhosseindhv]

Hey @aress31 , This pr was automatically closed because next branch is deleted. See #437 .

Notistack used to be tightly coupled with mui and use the theme object, but in favour of #421 and #184 we are not using the theme object anymore.

That doesn't mean you can't use mui theme or support dark theme yourself. Using notistack v3 (which is still in alpha), you can have custom components for each variant:

https://codesandbox.io/s/cold-brook-gsubs?file=/src/index.js