force vim background to black

 colorscheme wombat256
 highlight Normal guibg=black guifg=white
 set background=dark

Tweak to taste :)

@edit: after reading you later comment I suspect you'll find you need to override more related highlight (group) background colors. It'll be clearest which ones, by reading the existing color schemes

This is what worked for me:

colorscheme wombat256
hi Normal ctermbg=16 guibg=#000000
hi LineNr ctermbg=16 guibg=#000000