Typescript - Override MUIDataTableSelectCell not possible

Hey

I try to override the Table select cell but it is not possible since it does not exist in `overrides.d.ts`. The only classes that exists there are:

``` javascript
MuiTable: TableClassKey;
MuiTableBody: TableBodyClassKey;
MuiTableCell: TableCellClassKey;
MuiTableContainer: TableContainerClassKey;
MuiTableFooter: TableFooterClassKey;
MuiTableHead: TableHeadClassKey;
MuiTablePagination: TablePaginationClassKey;
MuiTableRow: TableRowClassKey;
MuiTableSortLabel: TableSortLabelClassKey;
```

This gives the behavior shown in the sandbox.

## Current Behavior
Should be able to override that class so I can change the color.

## Steps to Reproduce (for bugs)
https://codesandbox.io/s/muidatatables-custom-toolbar-6wue1?file=/index.js

I have added a codesandbox. It shows pretty good that the box does not turn green with the rest of the header.

## Your Environment

| Tech | Version |
|--------------|---------|
| Material-UI | ^4.9.2 |
| MUI-datatables | ^3.1.4 |
| React | ^16.13.1 |
| browser | ?? |
| etc | ?? |