This is my custom dark blue theme for Portainer, mostly, but not entirely, based on the excellent Nord color palette.
As requested in the Portainer thread I have created a repo for it, for others to contribute if they choose.
As I've previously mentioned in the above thread, I consider the theme only about 98% complete. While it's quite usable as is, there are a small handful of things I still want/need to style or tweak (but my time is tight these days), as well as possibly missing some things entirely, as I styled things along the way as I used Portainer and thus may have missed some things if I didn't come across them during my own usage.
There are rumblings of having this theme added right into Portainer as a selectable theme choice, but it will be a while before (if) that happens, so in the meantime, anyone is free to use and modify it as they wish; all I ask is that my name be left with it/derivatives of it, for the time spent on it (quite a few days, as I'm not a CSS poobah, so I also had to learn along the way).
Until it is (maybe) added directly into Portainer itself, you'll need to use a browser extension to enable this theme, such as Stylus or Stylish, and then paste this CSS into it as a new style.
Side Note
I got all of the selectors from the Chrome inspector; chances are likely a real CSS guru will spot ways to merge and optimize them; I welcome that.