I have a form with multiple FormControl (https://mui.com/api/form-control/) components and I have them wrapped in a:
const FormBody = styled(Box)(() => ({
display: 'flex',
flexDirection: 'column',
}));
<FormBody onSubmit={handleSubmitLogin(onLoginSubmit)} component="form">
<FormControl>
...
But I was hoping to create some CSS rules in the theme of MUI but I can't find any component that can be used to encapsulate the FormControl components.