let s:bg_state='dark'

function! ToggleBg()
        if s:bg_state0 == 'dark'
                hi Normal ctermbg=grey
                let s:bg_state='light'
        else
                hi Normal ctermbg=black
                let s:bg_state='dark'
        endif
endfunc

map <F5> :call ToggleBg()<CR>