feat: add toggle for email render mode and update config schema

[AidanWarner97]

This adds support for live switching between dark mode and light mode reading pane. This fixes multiple reports of issues with readability on different emails.

[bengotow]

Ahh this is great! Thanks for contributing this 🙌

Code-wise this looks solid, and it'll be really nice for people to have the option to toggle this.

Could we move the toggle down one layer so it sits next to Print? The top row of buttons in the toolbar are sort of "thread actions" and this is more of a "thread display/rendering" thing ala print/popout. I've been trying to avoid cluttering that top bar because I've heard some folks already find it a bit overwhelming.

It'd also be great to add a small option here to control this setting, just so it's repeated on the Appearance preferences. I think we could make it look like the "Tray icon for new messages" option and repeat the icons so people link the thread view icon to the setting.

[AidanWarner97]

Could we move the toggle down one layer so it sits next to Print? The top row of buttons in the toolbar are sort of "thread actions" and this is more of a "thread display/rendering" thing ala print/popout. I've been trying to avoid cluttering that top bar because I've heard some folks already find it a bit overwhelming.

That makes sense.

It'd also be great to add a small option here to control this setting, just so it's repeated on the Appearance preferences. I think we could make it look like the "Tray icon for new messages" option and repeat the icons so people link the thread view icon to the setting.

How would you prefer it? Just to allow people to customise the colour it lights up with?