useColorModeValue is a custom hook used to get a value from either of the parameters passed based on active color mode value.
import { useColorModeValue } from 'native-base';
function UseColorMode() {
  const {
    colorMode,
    toggleColorMode
  } = useColorMode();
  return <Center flex={1} bg={useColorModeValue('white', 'black')}>
      <Text fontSize="lg" display="flex">
        The active color mode is
        <Text bold>{useColorModeValue('Light', 'Dark')}</Text>
      </Text>
      <Button m={2} onPress={toggleColorMode}>
        Toggle
      </Button>
    </Center>;
}
function Example() {
  return <NativeBaseProvider>
      <UseColorMode />
    </NativeBaseProvider>;
}
Accepts 2 parameters and returns either of the two, based on current color-mode (first parameter for light mode and second parameter for dark mode).