Edit: I think I misunderstood. Although they are similar, "mørk" is the opposite to "light" or "bright", whereas "murk" and "murky" are more specific, usually to do with a mist or something similar making it hard to see.
I was marked incorrect for translating this as 'dark ale' rather than 'beer', which I think would be the natural English translation of this phrase. I drink real ale, quite often pale ales, but every now and again I do like a dark ale. I know we are told that ol is beer, but surely it also means ale.
Adjective endings depend on number, gender and definiteness: All definite adjectives end in -e. Indefinite common adjectives have no ending. Indefinite neuter adjectives end in -t. Indefinite plural adjectives end in -e.
In this case, øl is indefinite common, so mørk takes no ending.
Edit: Watch out for irregular adjectives like lille.