The default UI for Libertine is Grey. However, depending on certain lighting conditions, time of day or personal preference you might want to try some of the darker versions available. screenshots of all modes.
colorscheme libertine
let g:libertine_Sunset = 1
colorscheme libertine
let g:libertine_Twilight = 1
colorscheme libertine
let g:libertine_Midnight = 1
colorscheme libertine
let g:libertine_Pitch = 1
colorscheme libertine
development version here