some improvement on app-shell docs

[jodhi]

Hi all,
Just saw the docs related with app-shell missing theme var
link: https://mantine.dev/core/app-shell/#responsive-styles

maybe we should put this beforehand

const theme = useMantineTheme();

Thanks

[rtivital]

Yep, we are missing that, can you create a PR with a fix?

[jodhi]

of course, coming right up

[jodhi]

this is the PR #443