How can I add a background color to a <Container> element?

[limber sparrow]

<>
  <Flex direction={'row'}>
    <Container fluid >Hello</Container>
    <Container fluid >Hello</Container>
  </Flex>
</>```

I realize now I can use sx prop.

Is there any method that allows you to assign a class name to a component and address it traditionally in css instead?

I would prefer that method in this case I think, and I also need to add some flex properties to the element.