Add Window::set_theme (#2553)

* Add `Window::set_theme`

* typo

* fix linux build

* fix wayland

* review changes

* update docs

* update changelog

* pin `image` dep

* suppport falling back to system default

* fix linux

* default to dark on macOS and x11

* fix `setAppearance` definition

* add macOS notes

* update docs

* Update CHANGELOG.md

Co-authored-by: Markus Siglreithmaier <m.siglreith@gmail.com>

* update doc

* Revert "pin `image` dep"

This reverts commit 7517f7c506.

* Update theme example with Window::set_theme

* Fix Window::theme getter on macOS

Co-authored-by: Markus Siglreithmaier <m.siglreith@gmail.com>
Co-authored-by: Mads Marquart <mads@marquart.dk>

src/platform_impl/android/mod.rs

@@ -1041,6 +1041,8 @@ impl Window {
         self.app.content_rect()
     }
 
+    pub fn set_theme(&self, _theme: Option<Theme>) {}
+
     pub fn theme(&self) -> Option<Theme> {
         None
     }