You can always specify custom styles on a material-ui component by passing it the style property like so:

<AppBar style={{ margin: 0 }}/>

That will override the default root element style. If the property you're willing to change is on a children component, you'll have to set it using CSS, if there is no specific property material-ui exposes you.

Removing the margin on the body would also fix your problem

body {

  margin: 0;

}

Although you should usually use a CSS reset to avoid getting errors like these by integrating the following CSS snippet:

*, *:after, *:before {

  box-sizing: border-box;

  font: inherit;

  color: inherit;

  margin: 0;

  padding: 0;

  border: none;

  outline: none;

}