useBreakpointValue is a custom hook which returns the value for the current
breakpoint from the provided responsive values object. This hook also responds
to the window resizing, returning the appropriate value for the new window size.
The new variant and size props don't currently accept responsive values
(specified as objects or arrays), but useBreakpointValue is a good way to
achieve the same behavior.
The useBreakpointValue hook returns the value for the current breakpoint.