Dark Mode theme via Page Tweaks does not seem accessible via keyboard shortcuts unlike other menu items. My suggestion is to either make the Dark Theme via Page Tweaks accessible via macOS settings to create a custom shortcut, or else to simply add a keyboard shortcut for toggling on the Dark Theme (my suggestion is Opt+Ctrl+N similar to the default for the excellent Noir Safari extension).
This builds on (and potentially resolves) the several suggestions around built-in dark modes and dark reader issues e.g.
- https://orionfeedback.org/d/2084-built-in-dark-mode-for-sites/50
- https://orionfeedback.org/d/8203-built-in-dark-mode-for-ios-ipados
- https://orionfeedback.org/d/8574-suggestions-for-optimizing-dark-mode-usage
Users would be able to either set their own keyboard shortcut to toggle on the Dark theme or else use Opt+Ctrl+N to toggle it on or off, resulting in a page tweak for that site which both persists and is synced across a user's Orion instances.