Porting Airline Themes

Install vim-airline and vim-crystalline and any other plugins you need in your .vimrc:

call plug#begin()
Plug 'vim-airline/vim-airline'
Plug 'rbong/vim-crystalline'
call plug#end()

Then, set your vim-airline theme, usually by calling :AirlineTheme <theme> or :colorscheme <theme>.

Once your theme is loaded, run this to port your theme and copy it to the clipboard:

let @+ = crystalline#PortAirlineTheme('<theme>') | echo @+

The output should look similar to the following:

call crystalline#GenerateTheme({
      \ 'A':    [[17,  190], ['#00005f', '#dfff00'], 'bold'],
      \ 'B':    [[255, 238], ['#ffffff', '#444444'], ''],
      \ 'Fill': [[85,  234], ['#9cffd3', '#202020'], ''],
      \ })

If the theme has light and dark variants, you can run :set bg=light or :set bg=dark and repeat these steps.

Put the output in ~/.vim/autload/theme/my_theme.vim:

function! crystalline#theme#my_theme#SetTheme() abort
  if &background ==# 'dark'
    " crystalline#GenerateTheme({ ... })
  else
    " crystalline#GenerateTheme({ ... })
  endif
endfunction

Full example: the solarized vim-crystalline theme.