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