Keyboard shortcut support to toggle default/dark style

### Problem

We've received requests for a keyboard shortcut to toggle default/light visual style (https://github.com/elementary/switchboard-plug-keyboard/issues/398), and I think that makes sense—but we need something to actually listen for this keyboard shortcut (and contain the GSettings key to change it).

Apologies if this is the wrong place; I _think_ Gala makes sense since it would be listening to keyboard shortcuts, but feel free to move if not.

### Proposal

Gala should have a GSetting for a default/dark visual style toggle, then we can expose that in the Shortcuts overlay, System Settings, etc. We'd probably also read this setting from an indicator to show accelerators if we add a dark style toggle there.

### Prior Art (Optional)

_No response_