Why is this returning an error?
const btn = createMuiTheme({Button: {borderRadius: 100%},});
You have to change the Button to MuiButton.
Refer this link. https://mui.com/customization/theme-components/#css
const theme = createTheme({
components: {
// Name of the component
MuiButton: {
styleOverrides: {
// Name of the slot
root: {
// Some CSS
borderRadius: '100%'
},
},
},
},
});