Thanks for opening the issue!

For styleOverrides, I believe it should look like this:

const theme = createTheme({
  components: {
    MuiDataGrid: {
      styleOverrides: {
        actionsCellItem: {
          // ...
        }
      }
    }
  }
});

It doesn't work though, it's a bug.

For the defaultProps, from what I can tell you can only pass the props to the root component (DataGrid):

const theme = createTheme({
  components: {
    MuiDataGrid: {
      defaultProps: {
        // ...
      }
    }
  }
});

Hello @Nitzperetz
Data grid package has a somewhat different theme overriding approach to date-pickers if you are comparing the two by any chance.
As @cherniavskii mentioned, DataGrid only allows overriding both props and styles on the root element itself.
Do you have any particular use case where you would want certain defaultProps on GridActionsCellItem?

As for the style overrides, you can find the possible classes you can target here.
Here is an example of how it's possible to override actionsCell styles. On this you are correct. There is no actionsCellItem option, but as you can see—there is already a PR created by @cherniavskii to add it in.

You might also expect that applying the following change should produce the same result.

 const theme = createTheme({
   components: {
     MuiDataGrid: {
       styleOverrides: {
-        root: {
+        actionsCell: {
-          [`.${gridClasses.actionsCell}`]: {
             backgroundColor: "red"
-           }
         }
       }
     }
   }
 });

But that is not the case, because the given component isn't a styled one and does not support direct overrides.
Which is probably an issue on our side (@mui/xgrid), because based on typing support—you could expect that to work as well.

As with anything, it's best to explain your case with as clear examples as possible. It's always perfect if you provide a codesandbox with what is the end goal you are trying to achieve.

@LukasTy I think this should work:

styleOverrides: {
  actionsCell: {
    backgroundColor: "red"
  }
}

The same way as this works: https://codesandbox.io/s/tender-bird-vikzq5?file=/demo.tsx

styleOverrides: {
  row: {
    backgroundColor: "red"
  }
}

But that is not the case, because the given component isn't a styled one and does not support direct overrides

I don't think it's related to styled.
Those overrides only work if style overrides are handled in GridRootStyles - see https://github.com/mui/mui-x/pull/7635/files#r1084306896

But that is not the case, because the given component isn't a styled one and does not support direct overrides

I don't think it's related to styled.

Yes, sorry for not expressing myself correctly. For named style overrides to work they need to be consumed by any styled components' overridesResolver.
Here is a working example with your mentioned way of overriding it on your PR.

For named style overrides to work they need to be consumed by any styled components' overridesResolver.

Makes sense 👍
In this case, the GridRootStyles's overridesResolver consumes those overrides

Thank you all for replying!

As @cherniavskii mentioned, DataGrid only allows overriding both props and styles on the root element itself.
Do you have any particular use case where you would want certain defaultProps on GridActionsCellItem?

I am in need to align GridActionsCellItem component across the app.
For styling I wanted to change fontSize.
For PropsOverride the need is to set showInMenu to be true by default.