dotfiles/vimrc: fix truecolor support

I never really knew why the colors looked different inside and out
of tmux. Now they look correct everywhere!

See: https://gist.github.com/andersevenrud/015e61af2fd264371032763d4ed965b6

dotfiles/vimrc

@@ -11,10 +11,17 @@ filetype on             " enable file type detection
 filetype plugin on      " enable file type plugins
 filetype indent on      " enable file type indents
 
-set background=dark
-" restrict vim to 16 ANSI colors in the palette instead of approximating 256
-set t_Co=16
+" You might have to force true color when using regular vim inside tmux as the
+" colorscheme can appear to be grayscale with "termguicolors" option enabled.
+" See: https://gist.github.com/andersevenrud/015e61af2fd264371032763d4ed965b6
+if !has('gui_running') && &term =~ '^\%(screen\|tmux\)'
+  let &t_8f = "\<Esc>[38;2;%lu;%lu;%lum"    " set foreground color
+  let &t_8b = "\<Esc>[48;2;%lu;%lu;%lum"    " set background color
+endif
+
 colorscheme selenized
+set termguicolors
+set background=dark
 
 " for per-directory vimrc files
 " see: https://jezenthomas.com/how-i-write-invoices-in-vim/