Add support for Window::theme on the web

[tronical]

• Tested on all platforms changed
• Added an entry to CHANGELOG.md if knowledge of this change could be valuable to users
• Updated documentation to reflect any user-facing changes, including notes of platform-specific behavior
• Created or updated an example program if it would help users understand this functionality
• Updated feature matrix, if new features were added or implemented

[ogoffart]

I think this change is useful as is, so IMHO shouldn't prevent to be merged. But it could also emit the ThemeChanged event when the media query changes (using for example MediaQueryList::set_onchange)

[tronical]

I think this change is useful as is, so IMHO shouldn't prevent to be merged. But it could also emit the ThemeChanged event when the media query changes (using for example MediaQueryList::set_onchange)

Winit already implements the part of sending ThemeChanged when targeting the web. It's just missing the getter for the current value.

[madsmtm]

Only a minor nit with the changelog, otherwise looks fine

[tronical]

Good point, applied:). Feel free to choose merge with squash. (Or I can also force push a squash if you prefer)

[madsmtm]

Don't have a preference for how you do the commits, they'll be squashed once merged anyhow ;)

I'll need you to fix the conflicting changelog though, since I don't have write access to your fork.

[tronical]

Oops, not sure why that permission isn't there. I rebased, squashed, and resolved the conflict.

[madsmtm]

Thanks!