I would like to change style of checked thumb, but i cant figure how. I tried every variation.

When thumb is checked, it should change size to width: 20, height: 20 and color: white..

Can someone provide working example or change my code? Thanks!

const StyledSwitch = styled((props: SwitchProps) => (
      <Switch focusVisibleClassName=".Mui-focusVisible" disableRipple {...props} />
    ))(({}) => ({
      width: 42,
      height: 26,
      padding: 0,
      marginRight: -16,
      '& .MuiSwitch-switchBase': {
        padding: 3,
        margin: 2,
        transitionDuration: '300ms',
        '& .MuiSwitch-thumb': {
          boxSizing: 'border-box',
          width: 16,
          height: 16,
          boxShadow: 'none',
          backgroundColor: 'red',
        },
        '&.Mui-checked': {
          transform: 'translateX(16px)',
          color: '#fff',
          '& + .MuiSwitch-track': {
            backgroundColor: '#00519E',
            opacity: 1,
            border: '2px solid #00519E',
          },
          '&.Mui-disabled + .MuiSwitch-track': {
            opacity: 0.5,
          },
        },
        '&.Mui-disabled + .MuiSwitch-track': {
          opacity: 0.7,
        },
      },
    
      '& .MuiSwitch-track': {
        borderRadius: 26 / 2,
        backgroundColor: '#FFF',
        border: '2px solid #00519E',
        opacity: 1,
      },
    }));
    
    interface FormSwitchProps extends SwitchProps {
      name?: string;
    }
    
    const FormSwitch: React.FC<FormSwitchProps> = ({ name, ...props }) => (
      <FormControlLabel
        control={<StyledSwitch name={name} {...props} />}
        label={<span className="ml-1">{name}</span> || ''}
      />
    );
    
    export default FormSwitch;

>Solution :

To change the thumb size to width: 20, height: 20 and the color to white when checked, you should target the checked state of the thumb. Here’s the revised styling:

const StyledSwitch = styled((props: SwitchProps) => (
  <Switch focusVisibleClassName=".Mui-focusVisible" disableRipple {...props} />
))(({}) => ({
  width: 42,
  height: 26,
  padding: 0,
  marginRight: -16,
  '& .MuiSwitch-switchBase': {
    padding: 3,
    margin: 2,
    transitionDuration: '300ms',
    '& .MuiSwitch-thumb': {
      boxSizing: 'border-box',
      width: 16,
      height: 16,
      boxShadow: 'none',
      backgroundColor: 'red',
    },
    '&.Mui-checked': {
      transform: 'translateX(16px)',
      color: '#fff',
      '& .MuiSwitch-thumb': {      // Target the thumb when it's in a checked state
        width: 20,                 // Set the width to 20
        height: 20,                // Set the height to 20
        backgroundColor: 'white',  // Set the color to white
      },
      '& + .MuiSwitch-track': {
        backgroundColor: '#00519E',
        opacity: 1,
        border: '2px solid #00519E',
      },
      '&.Mui-disabled + .MuiSwitch-track': {
        opacity: 0.5,
      },
    },
    '&.Mui-disabled + .MuiSwitch-track': {
      opacity: 0.7,
    },
  },
  '& .MuiSwitch-track': {
    borderRadius: 26 / 2,
    backgroundColor: '#FFF',
    border: '2px solid #00519E',
    opacity: 1,
  },
}));

interface FormSwitchProps extends SwitchProps {
  name?: string;
}

const FormSwitch: React.FC<FormSwitchProps> = ({ name, ...props }) => (
  <FormControlLabel
    control={<StyledSwitch name={name} {...props} />}
    label={<span className="ml-1">{name}</span> || ''}
  />
);

export default FormSwitch;

The key here is to nest & .MuiSwitch-thumb inside the &.Mui-checked selector to properly target the thumb when the switch is checked.