256 color terminal support

[GordianDziwis]

Describe the bug

We have this function to override the terminal color codes in case of a 256 color terminal. But where are those color codes are actually from?

" `base16_colorspace` and `base16colorspace` are legacy properties and
" exist to keep existing setups from breaking
if (exists('base16_colorspace') && base16_colorspace ==? '256') || (exists('tinted_colorspace') && tinted_colorspace ==? '256')
  let s:cterm01        = '18'
  let g:tinted_cterm01 = '18'
  let s:cterm02        = '19'
  let g:tinted_cterm02 = '19'
  let s:cterm04        = '20'
  let g:tinted_cterm04 = '20'
  let s:cterm06        = '21'
  let g:tinted_cterm06 = '21'
  let s:cterm09        = '16'
  let g:tinted_cterm09 = '16'
  let s:cterm0F        = '17'
  let g:tinted_cterm0F = '17'
  let s:cterm10        = '13'
  let g:tinted_cterm10 = '13'
  let s:cterm11        = '13'
  let g:tinted_cterm11 = '13'
  let s:cterm12        = '09'
  let g:tinted_cterm12 = '09'
  let s:cterm13        = '11'
  let g:tinted_cterm13 = '11'
  let s:cterm14        = '10'
  let g:tinted_cterm14 = '10'
  let s:cterm15        = '14'
  let g:tinted_cterm15 = '14'
  let s:cterm16        = '12'
  let g:tinted_cterm16 = '12'
  let s:cterm17        = '13'
  let g:tinted_cterm17 = '13'
else
...

[JamyGolden]

I'm not sure why there is a mismatch between the cterm numbers and the numbers given, but my understanding is that if you use set notermguicolors it uses the ANSI colors from the terminal for the colors.