Looks fine to me; wild that anyone would think it unserious. Design is much more about how it works than following the latest graphic trends.
I think the design issue is more of an HN meme these days, than an actual problem.
That said, I rather liked what Thunderbird and JetBrains did - let the user choose not only the fonts and stuff like that, but also the UI density that they want.
We're also planning to give a wider array of options in an upcoming release, including independently setting the line-height and the font size. It's a lot of work to get even two options to both have a reasonable layout throughout the UI, though (among other things, it involved changing a lot of hardcoded values in px to be in relative units), which is why only the two-way switch made it into this week's release.