While I emphatically agree that Hanners looks good in her dress - and it is a new outfit for her - does this honestly count as her massively dressing up? I mean that dress isn't exactly a ball gown. Yeah, it's a bigger effort than we've usually seen on her part, but still, it's just a dress. That's a normal thing to wear, those are normal shoes, that's a normal purse. A very pretty-looking combination thereof, and a notch above her usual outfits, but it's not like she went to the top end of town to get specially fitted for this. Seriously. Hanners normally wears, what, a shirt and jeans? Just like every other female character mostly does? And if we slapped any other character into a nice-looking dress for a date, we'd comment that it was a bit fancier, but it's not a huge step out.
My point being that while she's made an effort to look pretty, she's not all that fancied up. Which means that Sven wearing a hoodie isn't exactly a disaster scenario. He's dressed slightly more casual than her, no big harm.
Hell, you want to talk about clothing conspiracies, try this on for size. I've always put Sven in the role of being a mirror character for Marten; Hannelore has shown a borderline obsession with Marten in the past, including her first-ever flirt; Marten's most common attire is his iconic black hoodie. So when Marten's #1 fangirl goes on a date with the anti-Marten, what's he wear? Why, the photo-opposite of what Marten himself would wear!
Intentional juxtaposition, or Freudian slip on behalf of Jeph when he picked Sven's outfit? You decide.