That would be "stippi" aka. Stephan Aßmus on both counts, I believe. At least the icon style was chosen after a competition all the way back in 2006, where stippi's style got the most votes [1]. The look & feel of controls was done by him in 2009 [2], not sure if there was ever an announcement about that (I see an old forum thread that may be related, though.)

These days, stippi does not have much time for Haiku, but there are a number of developers & community members who have picked up where he left off, continuing work on the UI and drawing new icons when required.

(Old hands from the BeOS days may remember stippi as the co-developer of the shareware "WonderBrush", which now runs on Haiku and has been open-sourced [3].)

[1]: https://www.haiku-os.org/news/2006-11-03_icon_contest_and_be...

[2]: https://github.com/haiku/haiku/commit/2f86ba45579bdc9648b232...

[3]: https://github.com/stippi/WonderBrush-v2/

Where do you even see them? I clicked around the website for a minute and can't find a freaking screenshot of anything anywhere. How frustrating.

I can't speak to the specific person(s) however the Haiku inc. (The non-profit for Haiku Org stuff) has an FAQ entry about them: https://www.haiku-inc.org/trademarks/haiku_icons/

Edit: Fixed typo