<GlobalStyles />

Sometimes you might want to insert global css. You can use the <GlobalStyles /> component to do this.

It's styles (with an s) prop should be of same type as the css() function argument.

import { GlobalStyles } from "tss-react";
import { useStyles } from "tss-react/mui";

function MyComponent() {

    const { theme } = useStyles();

    return (
        <>
            <GlobalStyles
                styles={{
                    "body": {
                        "backgroundColor": theme.palette.background.default,
                    },
                    ".foo": {
                        "color": "cyan"
                    },
                }}
            />
            <h1 className="foo">This text will be cyan</h1>
        </>
    );
}

Is there a reason to use this instead of import GlobalStyles from "@mui/material/GlobalStyles";? No