How do you change a style of a child when hovering over a parent using Material UI styles?

Below is an example of the correct syntax for v4 ("& $addIcon" nested within &:hover). Further down are some v5 examples.

import * as React from "react";
import { render } from "react-dom";
import { Grid, makeStyles } from "@material-ui/core";
import AddIcon from "@material-ui/icons/Add";

const useStyles = makeStyles(theme => ({
  outerDiv: {
    backgroundColor: theme.palette.grey[200],
    padding: theme.spacing(4),
    '&:hover': {
      cursor: 'pointer',
      backgroundColor: theme.palette.grey[100],
      "& $addIcon": {
        color: "purple"
      }
   }
  },
  addIcon: (props: { dragActive: boolean }) => ({
    height: 50,
    width: 50,
    color: theme.palette.grey[400],
    marginBottom: theme.spacing(2)
  })
}));

function App() {
  const classes = useStyles();
  return (
    <Grid container>
      <Grid item className={classes.outerDiv}>
        <AddIcon className={classes.addIcon} />
      </Grid>
    </Grid>
  );
}

const rootElement = document.getElementById("root");
render(<App />, rootElement);

Related documentation and answers:

• https://cssinjs.org/jss-plugin-nested?v=v10.0.0#use-rulename-to-reference-a-local-rule-within-the-same-style-sheet
• how to use css in JS for nested hover styles, Material UI
• Material UI: affect children based on class
• Advanced styling in material-ui

---

For those who have started using Material-UI v5, the example below implements the same styles but leveraging the new sx prop.

import Grid from "@mui/material/Grid";
import { useTheme } from "@mui/material/styles";
import AddIcon from "@mui/icons-material/Add";

export default function App() {
  const theme = useTheme();
  return (
    <Grid container>
      <Grid
        item
        sx={{
          p: 4,
          backgroundColor: theme.palette.grey[200],
          "&:hover": {
            backgroundColor: theme.palette.grey[100],
            cursor: "pointer",
            "& .addIcon": {
              color: "purple"
            }
          }
        }}
      >
        <AddIcon
          className="addIcon"
          sx={{
            height: "50px",
            width: "50px",
            color: theme.palette.grey[400],
            mb: 2
          }}
        />
      </Grid>
    </Grid>
  );
}

---

Here’s another v5 example, but using Emotion’s styled function rather than Material-UI’s sx prop:

import Grid from "@mui/material/Grid";
import { createTheme, ThemeProvider } from "@mui/material/styles";
import AddIcon from "@mui/icons-material/Add";
import styled from "@emotion/styled/macro";

const StyledAddIcon = styled(AddIcon)(({ theme }) => ({
  height: "50px",
  width: "50px",
  color: theme.palette.grey[400],
  marginBottom: theme.spacing(2)
}));
const StyledGrid = styled(Grid)(({ theme }) => ({
  padding: theme.spacing(4),
  backgroundColor: theme.palette.grey[200],
  "&:hover": {
    backgroundColor: theme.palette.grey[100],
    cursor: "pointer",
    [`${StyledAddIcon}`]: {
      color: "purple"
    }
  }
}));
const theme = createTheme();
export default function App() {
  return (
    <ThemeProvider theme={theme}>
      <Grid container>
        <StyledGrid item>
          <StyledAddIcon />
        </StyledGrid>
      </Grid>
    </ThemeProvider>
  );
}

---

And one more v5 example using Emotion’s css prop:

/** @jsxImportSource @emotion/react */
import Grid from "@mui/material/Grid";
import { createTheme, ThemeProvider } from "@mui/material/styles";
import AddIcon from "@mui/icons-material/Add";

const theme = createTheme();
export default function App() {
  return (
    <ThemeProvider theme={theme}>
      <Grid container>
        <Grid
          item
          css={(theme) => ({
            padding: theme.spacing(4),
            backgroundColor: theme.palette.grey[200],
            "&:hover": {
              backgroundColor: theme.palette.grey[100],
              cursor: "pointer",
              "& .addIcon": {
                color: "purple"
              }
            }
          })}
        >
          <AddIcon
            className="addIcon"
            css={(theme) => ({
              height: "50px",
              width: "50px",
              color: theme.palette.grey[400],
              marginBottom: theme.spacing(2)
            })}
          />
        </Grid>
      </Grid>
    </ThemeProvider>
  );
}