Add type for theme

[djhi]

Fixes #5426

This is only the beginning. Indeed there is a lot of work in order to provide type safe overrides.
See how MUI does it: https://github.com/mui-org/material-ui/blob/master/packages/material-ui/src/styles/overrides.d.ts#L103

I also wonder if our approach for the Sidebar size and closedSize configuration is correct. Maybe we should just rely on props with defaults (which can be overridden through the theme as well (https://material-ui.com/customization/globals/#default-props). This would lead to a single entry to customize the sidebar instead of two (sidebar at the theme root and RaSidebar in the overrides)

We would have to export the class keys for every component like this:

export type SidebarClassKey = keyof ReturnType<typeof useStyles>;

This could be a very good opportunity for first time contributors