Styling the colums header

[Yitavg]

Hi,
I have an issue styling the column header of the react table.
I used the following prop:

  muiTableHeadCellProps={{
          sx: {
            background: backgroundColor,
            border: "2px solid red" 
          },

and got this result:

As you can see, there is white background on each side between the header and the border.
What prop do I use to overwrite it? I literally tried every prop in the docs.
thanks

[KevinVandy]

The code is correct. You're pretty much following this example. I'm guessing you just passed an invalid color in.