Use the Flex component to create flexbox layouts.
Flex
import { Flex } from 'theme-ui'
<Flex> <Box p={2} bg="primary" color="white" sx={{ flex: '1 1 auto' }}> Flex </Box> <Box p={2} bg="muted"> Box </Box> </Flex><Flex> <Box p={2} bg="primary" color="white" sx={{ flex: '1 1 auto' }}> Flex </Box> <Box p={2} bg="muted"> Box </Box></Flex>
<Flex> <Box p={2} bg="primary" color="white" sx={{ flex: '1 1 auto' }}> Flex </Box> <Box p={2} bg="muted"> Box </Box></Flex>