tooltip.js

...
import { AreaChart, Tooltip, Area } from "recharts";
...
...
<AreaChart data={dataArr} height={250} width={700}>
  <Tooltip
    contentStyle={styles.tooltipWrapper}
    labelStyle={styles.tooltip}
    formatter={value => `${value}`}
  />
  <Area
    dataKey="Total"
    stroke="none"
    fillOpacity={1}
    fill="#f7931a"
  />
</AreaChart>
...
const styles = {
  container: {
    maxWidth: 700,
    margin: "0 auto"
  },
  tooltipWrapper: {
    background: "#444444",
    border: "none"
  },
  tooltip: {
    color: "#ebebeb"
  }
};