Adding custom breakpoint

[Bersaelor]

Hello, I'd like to add a custom breakpoint like:

const isMobile = `@media only screen and (max-width: 600px), (max-height: 600px)`;
const xs = useMediaQuery(isMobile);
// not just const xs = useMediaQuery(useTheme().breakpoints.down('sm'));

so I can match phones in both landscape and portrait.
Now, I also like to use <Grid item xs={12} sm={3}> when working in MUI, which of course then triggers at different screen sizes then the above simple query would.

Did anyone encounter similar situations?
Do I have to do the grid-sizing without the convenient breakpoints now?

[juanrgm]

#170 (comment)