Fix style overrides for Ra* components

[sweco-sedalh]

This pull request allows using styleOverrides from MUI themes to override the styles of all classes in react-admin components, as opposed to just the root class previously.

Fixes: #7604

[djhi]

Almost there! Sorry I did not see the last point that immediately

[djhi]

Have you tried those modifications? I tried converting the demo theme accordingly and it does not seem to work

[sweco-sedalh]

Have you tried those modifications? I tried converting the demo theme accordingly and it does not seem to work

I have tested them by using basically the same code as in the sandbox on #7604 applied to the simple example:

const theme = _.merge({}, defaultTheme, {
    components: {
      MuiButton: {
        styleOverrides: {
          root: {
            // works as expected
            border: "5px solid red"
          },
          startIcon: {
            // works as expected
            color: "red"
          }
        }
      },
      RaList: {
        styleOverrides: {
          root: {
            // works as expected
            backgroundColor: "green"
          },
          content: {
            // does not work
            backgroundColor: "red"
          }
        }
      }
    }
  });

I hadn't seen the demo theme before, will try to update it as well (probably should be part of this PR).

[sweco-sedalh]

Now the demo theme is updated and works.

[fzaninotto]

Thanks for your contribution. But I'm -1 for this change, for the reasons I explained in #7604.

[fzaninotto]

As explained in #7604, we won't merge that change in master. Thanks for your contribution anyway!