Write documentation for new ThemeFn traits

src/application.rs

@@ -530,9 +530,19 @@ where
     }
 }
 
-/// TODO
+/// The theme logic of some [`Application`].
+///
+/// Any implementors of this trait can be provided as an argument to
+/// [`Application::theme`].
+///
+/// `iced` provides two implementors:
+/// - the built-in [`Theme`] itself
+/// - and any `Fn(&State) -> impl Into<Option<Theme>>`.
 pub trait ThemeFn<State, Theme> {
-    /// TODO
+    /// Returns the theme of the [`Application`] for the current state.
+    ///
+    /// If `None` is returned, `iced` will try to use a theme that
+    /// matches the system color scheme.
     fn theme(&self, state: &State) -> Option<Theme>;
 }
 

src/daemon.rs

@@ -317,9 +317,19 @@ where
     }
 }
 
-/// TODO
+/// The theme logic of some [`Daemon`].
+///
+/// Any implementors of this trait can be provided as an argument to
+/// [`Daemon::theme`].
+///
+/// `iced` provides two implementors:
+/// - the built-in [`Theme`] itself
+/// - and any `Fn(&State, window::Id) -> impl Into<Option<Theme>>`.
 pub trait ThemeFn<State, Theme> {
-    /// TODO
+    /// Returns the theme of the [`Daemon`] for the current state and window.
+    ///
+    /// If `None` is returned, `iced` will try to use a theme that
+    /// matches the system color scheme.
     fn theme(&self, state: &State, window: window::Id) -> Option<Theme>;
 }