I’m not aware of any way to use browser extensions designed for Safari in Orion; the browser’s main claim to fame is that it supports extensions for both Mozilla Firefox and Google Chrome.
I would have to say that both dark mode & ad blocking are eminently possible on Orion as things are.
For ad blocking, I would recommend the Firefox version of uBlock Origin;
https://addons.mozilla.org/en-US/firefox/addon/ublock-origin/
For dark mode, see if the extension Dark Reader is acceptable to you;
https://chromewebstore.google.com/detail/dark-reader/eimadpbcbfnmbkopoojfekhnkhdbieeh?hl=en
Don’t forget to enable Firefox and Chrome extensions;
- Tap on the three dots in the bottom-right corner
- Tap on Settings
- Tap on Advanced
- Toggle on both Chrome Extensions and Firefox Extensions