Exactly, there is nothing dark about it. Gritty? Yes, but then for the vast majority of people life is gritty, and the song shows how even the poor and downtrodden find solace in love.
Myself, I always did find it uplifting because, for me, it was about two people at Xmas who are ripping each other to shreds ... but, nonetheless, still *were* together at Xmas. That was the main thing, for me. I might have been wrong about 'dark', fair enough: (The Pogues weren't really 'dark', I'd say - not as in 'black' and 'miserable', anyway - they were too into the craic for that) but if only 'gritty', it's a very, *very* gritty song. For instance, insofar as it's a story, it could (should?) be read as about Shane MacGowan being shoved in the drunk tank by NY cop (his real-life friend Matt Dillon) and reminiscing, in his drunken stupor, about his relationship with Kirsty MacColl. Then there are the lines, towards the end, "Happy Christmas your arse, I pray God it's our last".
Anyway - I'll declare an interest (and slot in some *super*-impressive name-dropping in the process ): My younger bro went out with the sister of Andrew Rankin, the Pogues' drummer, for about a year. So I have an extra little reason for being very fond of them as well as that song in particular. Till 1987 when 'Fairytale' came out they'd had hits, but nothing that made them properly 'big time' - in their own eyes, at least. Elvis Costello had challenged them to do an Xmas song that would make it, but at the same time not lead to them 'selling out' in favour of the giant demographic that required slush at Xmas. It looks like the song will endure for eternity - I hope so. I liked them, despite their being world-class pissheads who were always in danger of sinking their careers as a result.