feat: add system theme mode

[lyallcooper]

Instead of just having a light/dark mode toggle, most apps allow the theme to follow the system theme. This change adds a third state to the existing theme toggle button—system, represented by a monitor icon—that makes the app's theme follow that of the OS.

Also adds an event that fires when the system theme changes so that the app changes its theme accordingly without requiring a page refresh.

[CLAassistant]

All committers have signed the CLA.

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