egui-winit: Automatically detect and apply dark or light mode

Closes #1001

Uses the `dark-light` crate: https://crates.io/crates/dark-light

eframe/CHANGELOG.md

@@ -7,6 +7,7 @@ NOTE: [`egui_web`](egui_web/CHANGELOG.md), [`egui-winit`](egui-winit/CHANGELOG.m
 ## Unreleased
 * The default native backend is now `egui_glow` (instead of `egui_glium`) ([#1020](https://github.com/emilk/egui/pull/1020)).
 * The default web painter is now `egui_glow` (instead of WebGL) ([#1020](https://github.com/emilk/egui/pull/1020)).
+* Automatically detect and apply dark or light mode from system ([#1045](https://github.com/emilk/egui/pull/1045)).
 
 
 ## 0.16.0 - 2021-12-29

egui-winit/CHANGELOG.md

@@ -4,6 +4,7 @@ All notable changes to the `egui-winit` integration will be noted in this file.
 
 
 ## Unreleased
+* Automatically detect and apply dark or light mode from system ([#1045](https://github.com/emilk/egui/pull/1045)).
 * Replaced `std::time::Instant` with `instant::Instant` for WebAssembly compatability ([#1023](https://github.com/emilk/egui/pull/1023))
 
 

egui-winit/Cargo.toml

@@ -29,14 +29,15 @@ winit = "0.26"
 epi = { version = "0.16.0", path = "../epi", optional = true }
 
 copypasta = { version = "0.7", optional = true }
+dark-light = { version = "0.1.1", optional = true } # detect dark mode system preference
 serde = { version = "1.0", optional = true, features = ["derive"] }
 webbrowser = { version = "0.5", optional = true }
 
 # feature screen_reader
 tts = { version = "0.19", optional = true }
 
 [features]
-default = ["clipboard", "links"]
+default = ["clipboard", "dark-light", "links"]
 
 # enable cut/copy/paste to OS clipboard.
 # if disabled a clipboard will be simulated so you can still copy/paste within the egui app.

egui-winit/src/epi.rs

@@ -206,22 +206,29 @@ impl EpiIntegration {
         persistence: crate::epi::Persistence,
         app: Box<dyn epi::App>,
     ) -> Self {
-        let egui_ctx = egui::CtxRef::default();
-
-        *egui_ctx.memory() = persistence.load_memory().unwrap_or_default();
+        let prefer_dark_mode = prefer_dark_mode();
 
         let frame = epi::Frame::new(epi::backend::FrameData {
             info: epi::IntegrationInfo {
                 name: integration_name,
                 web_info: None,
-                prefer_dark_mode: None, // TODO: figure out system default
+                prefer_dark_mode,
                 cpu_usage: None,
                 native_pixels_per_point: Some(crate::native_pixels_per_point(window)),
             },
             output: Default::default(),
             repaint_signal,
         });
 
+        let egui_ctx = egui::CtxRef::default();
+        *egui_ctx.memory() = persistence.load_memory().unwrap_or_default();
+
+        if prefer_dark_mode == Some(true) {
+            egui_ctx.set_visuals(egui::Visuals::dark());
+        } else {
+            egui_ctx.set_visuals(egui::Visuals::light());
+        }
+
         let mut slf = Self {
             frame,
             persistence,
@@ -311,3 +318,16 @@ impl EpiIntegration {
             .save(&mut *self.app, &self.egui_ctx, window);
     }
 }
+
+#[cfg(feature = "dark-light")]
+fn prefer_dark_mode() -> Option<bool> {
+    match dark_light::detect() {
+        dark_light::Mode::Dark => Some(true),
+        dark_light::Mode::Light => Some(false),
+    }
+}
+
+#[cfg(not(feature = "dark-light"))]
+fn prefer_dark_mode() -> Option<bool> {
+    None
+}

egui_glium/CHANGELOG.md

@@ -3,6 +3,7 @@ All notable changes to the `egui_glium` integration will be noted in this file.
 
 
 ## Unreleased
+* Automatically detect and apply dark or light mode from system ([#1045](https://github.com/emilk/egui/pull/1045)).
 
 
 ## 0.16.0 - 2021-12-29

egui_glium/Cargo.toml

@@ -24,7 +24,7 @@ all-features = true
 
 [dependencies]
 egui = { version = "0.16.0", path = "../egui", default-features = false, features = ["single_threaded"] }
-egui-winit = { version = "0.16.0", path = "../egui-winit", default-features = false, features = ["epi"] }
+egui-winit = { version = "0.16.0", path = "../egui-winit", default-features = false, features = ["dark-light", "epi"] }
 epi = { version = "0.16.0", path = "../epi", optional = true }
 
 glium = "0.31"

egui_glow/CHANGELOG.md

@@ -3,6 +3,7 @@ All notable changes to the `egui_glow` integration will be noted in this file.
 
 
 ## Unreleased
+* Automatically detect and apply dark or light mode from system ([#1045](https://github.com/emilk/egui/pull/1045)).
 
 
 ## 0.16.0 - 2021-12-29

egui_glow/Cargo.toml

@@ -30,7 +30,7 @@ glow = "0.11"
 memoffset = "0.6"
 
 [target.'cfg(not(target_arch = "wasm32"))'.dependencies]
-egui-winit = { version = "0.16.0", path = "../egui-winit", default-features = false, features = ["epi"], optional = true }
+egui-winit = { version = "0.16.0", path = "../egui-winit", default-features = false, features = ["dark-light", "epi"], optional = true }
 glutin = { version = "0.28.0", optional = true }
 
 [target.'cfg(target_arch = "wasm32")'.dependencies]