The developer in question specifically declined remuneration - he enjoyed his day job and did not want to quit it, and just was happy to share both his code (Tesseract) and instance (dubvee.org) with anyone, completely free of charge.

https://dubvee.org/about#donations :

I built Tesseract in my spare time as a hobby, learning excercise for Svelte, and to address my own personal annoyances with Lemmy and other Lemmy UIs. That said, I do not feel the need to accept donations for its development.

However, if you really want to donate, please consider donating to one of the following:

Xylight: They are the author of Photon from which Tesseract was forked who also did a lot of the heavy lifting for much of the core. While I've replaced a lot of it, none of my work would have been possible without theirs.