Customizing CSS styles not working as described

We tried to override the cell layout as described in the docs by naming the css file react_spreadsheet_grid_overrides.css. Using it in an serverside react app we did not get our custom css working without using !important

• react-spreadsheet-grid version: ^2.0.0
• React version: ^16.12.0
• Operating System: any

Relevant code or config

_app.js

  <link
            rel='stylesheet'
            type='text/css'
            href='/css/react_spreadsheet_grid_overrides.css'
          />

index.js

 const SheetExample2 = dynamic(
    () => import('../../components/SheetExample2'),
    {
      ssr: false
    }
  );

SheetExample2.jsx

import React, { useState, useRef } from 'react';
import { Grid as Table, Input, Select } from 'react-spreadsheet-grid';
import { IconButton, Button, Grid, makeStyles } from '@material-ui/core';
import DeleteForeverIcon from '@material-ui/icons/DeleteForever';
import AddCircleIcon from '@material-ui/icons/AddCircle';

const useStyles = makeStyles((theme) => ({
  gridAddIconButtonWrapper: { marginBottom: '5px' },
  gridDeleteButtonWrapper: { paddingTop: '40px' },
  addIcon: { color: '#008000' },
  deleteIcon: { color: '#FF0000' },
}));

const rowTemplate = { from: '', to: '', distance: '', fuel: '' };

const SheetExample2 = () => {
  const classes = useStyles();

  const rowIdIndex = useRef(1);
  const [rows, setRows] = useState([]);
  const [gridHeight, setGridHeight] = useState(100);

  const addNewRow = () => {
    setRows([...rows, { id: rowIdIndex.current, ...rowTemplate }]);
    setGridHeight((previousState) => previousState + 90);
    rowIdIndex.current++;
  };

  const onFieldChange = (rowId, field) => (value) => {
    const row = rows.find((row) => row.id === rowId);
    row[field] = value;
    setRows([].concat(rows));

    //calculation example for fuel, distance...
    if (row.from != null && row.to != null && row.from !== '' && row.to !== '') {
      row['distance'] = `${Math.floor(Math.random() * 100) + 100} km`;
      row['fuel'] = `${Math.floor(Math.random() * 100) + 50} L`;
    }
  };

  const deleteRow = (rowId) => {
    const remainedRows = rows.filter((row) => row.id !== rowId);
    setRows(remainedRows);
    setGridHeight((previousState) => previousState - 90);
  };

  const handleClick = (row, columnId) => {
    if (columnId === 'delete') {
      const remainedRows = rows.filter((r) => r.id !== row.id);
      setRows(remainedRows);
    }
  };

  return (
    <Grid container>
      <Grid item xs={12}>
        <IconButton variant='contained' onClick={addNewRow}>
          <AddCircleIcon fontSize='large' className={classes.addIcon} />
        </IconButton>
      </Grid>
      <Grid item xs={12}>
        <Table
          rowHeight={70}
          onCellClick={(row, columnId) => handleClick(row, columnId)}
          focusOnSingleClick={true}
          columns={[
            {
              id: 1,
              title: () => 'Start',
              value: (row, { focus }) => {
                return <Input value={row.from} onChange={onFieldChange(row.id, 'from')} focus={focus} />;
              },
            },
            {
              id: 2,
              title: () => 'End',
              value: (row, { focus }) => {
                return <Input value={row.to} onChange={onFieldChange(row.id, 'to')} focus={focus} />;
              },
            },
            {
              id: 3,
              title: () => 'Distance',
              value: (row, { focus }) => {
                return <Input value={row.distance} onChange={onFieldChange(row.id, 'distance')} focus={focus} />;
              },
            },
            },
            {
              id: 'delete',
              title: () => 'Action',
              value: () => {
                return (
                  <IconButton variant='contained' color='primary'>
                    <DeleteForeverIcon fontSize='large' className={classes.deleteIcon} />
                  </IconButton>
                );
              },
            },
          ]}
          rows={rows}
          getRowKey={(row) => {
            row.id;
          }}
        />
      </Grid>
    </Grid>
  );
};
export default SheetExample2;

react_spreadsheet_grid_overrides.css

...
.SpreadsheetGrid__headCell {
  display: inline-flex;
  position: relative;
  color: white;
  font-size: 12px;
  line-height: 14px;
  font-weight: 500;
  white-space: nowrap;
  border-bottom: none;
  padding: 10px 8px 8px 8px;
  text-align: left;
  background-color: #000000;
  align-items: center;
  border-bottom: 1px solid rgba(0, 0, 0, 0.12);
  border-right: 1px solid rgba(0, 0, 0, 0.12);
}
...

package.json

{
  "name": "client",
  "version": "1.0.0",
  "description": "SheetViewExample",
  "main": "index.js",
  "scripts": {
    "test": "echo \"Error: no test specified\" && exit 1",
    "dev": "nodemon index.js",
    "build": "next build",
    "start": "next start"
  },
  "keywords": [],
  "author": "",
  "license": "ISC",
  "dependencies": {
    "@material-ui/core": "^4.7.1",
    "@material-ui/icons": "^4.5.1",
    "@material-ui/lab": "^4.0.0-alpha.34",
    "async": "^3.1.1",
    "axios": "^0.19.0",
    "base-64": "^0.1.0",
    "express": "^4.17.1",
    "is-number": "^7.0.0",
    "isomorphic-unfetch": "^3.0.0",
    "js-cookie": "^2.2.1",
    "jwt-decode": "^2.2.0",
    "konva": "^4.0.18",
    "lodash": "^4.17.15",
    "next": "^9.1.4",
    "next-cookies": "^1.1.3",
    "next-i18next": "^0.52.1",
    "nodemon": "^1.19.4",
    "perf_hooks": "0.0.1",
    "pigeon-maps": "^0.14.0",
    "pigeon-marker": "^0.3.4",
    "pigeon-overlay": "^0.2.3",
    "react": "^16.12.0",
    "react-countdown-now": "^2.1.2",
    "react-dom": "^16.12.0",
    "react-draggable": "^4.2.0",
    "react-konva": "^16.9.0-1",
    "react-spreadsheet-grid": "^2.0.0",
    "seedrandom": "^3.0.5",
    "socket.io-client": "^2.3.0",
    "use-image": "^1.0.5"
  },
  "repository": {
    "type": "git",
    "url": ""
  }
}

The changes in our css were not applied until we used !important after every css property.