Little script for dimming/brightening screen colors

Not an issue

Working late at night I like adjusting the screen brightness/contrast. Perhaps someone else will also find this little mapping useful.

On Mac F1/F2 special keys (pressed together with Fn key) Dim/Brighten screen so this mapping seems very natural.

```
" brighten/dim background - a'la macOS dim screen function keys
" 233 (darkest) ~ 239 (lightest) 252 (darkest) ~ 256 (lightest)
function! Seoul256Brighten()
if g:seoul256_background == 239
let g:seoul256_background = 252
elseif g:seoul256_background == 256
let g:seoul256_background = 256
else
let g:seoul256_background += 1
endif
colo seoul256
endfunction
"
function! Seoul256Dim()
if g:seoul256_background == 252
let g:seoul256_background = 239
elseif g:seoul256_background == 233
let g:seoul256_background = 233
else
let g:seoul256_background -= 1
endif
colo seoul256
endfunction
"
nmap :call Seoul256Dim()
nmap :call Seoul256Brighten()
```