Everyone thinking about 2-state vs. 3-state controls is automatically honoring the device's color scheme setting. The controls are an extra to let users choose to not use the device setting on a particular site.

Rather than each site making its own design for such a control, I wish browsers would make it a per-site setting users could use.

That's basically @media (prefers-color-scheme: dark).

However, we should go a step further and provide a framework of UI elements that can be used consistently to create a uniform and accessible interface and they could even be pre-compiled into native controls for more efficient...hold on a gosh darn minute...