Add automatic theme (syncs with system settings)

[Bleuzen]

Hi :)
This adds an option to follow the systems color theme. If theme is changed in system settings, HTTP Toolkit will automatically adapt (if enabled).

[CLAassistant]

All committers have signed the CLA.

[pimterry]

Hi @Bleuzen, thanks for this! Great idea.

The underlying model all looks great to me, but I'm not totally sure about the UX. Was there a reason you didn't add this as another tab in the theme options? Most theme pickers I've seen elsewhere use Auto/Dark/Light for example. I think that might be a little clearer and more intuitive for users coming from other tools.

Firefox for example (with a system dark mode default):

What do you think?

[Bleuzen]

Was there a reason you didn't add this as another tab in the theme options?

Was, yeah. Had it as tab first but it was buggy sometimes under circumstances.
Note: I usually don't work with some stuff used in httptoolkit, so I probably just made a weird mistake with mobx or something.

Anyway, gave it another try and it works this time. Pushed now.

[Bleuzen]

I'm not perfectly happy with the icon yet, didn't find a simple one for automatic and adjust is already used for the high contrast theme, so went with magic for now.
@pimterry if you have a better idea, go ahead.

[pimterry]

Great stuff, thanks @Bleuzen! Looks perfect to me 👍

By the way, in case you're not aware, HTTP Toolkit Pro is totally free for contributors. If you're interested, just let me know your email address (here or send me a message at tim @ httptoolkit.com) and I'll set you up.