Introduce `auto-detect-theme` feature

2 files changed, 29 insertions, 2 deletions

diff --git a/core/Cargo.toml b/core/Cargo.toml
index c273fcb4..29a95ad7 100644
--- a/core/Cargo.toml
+++ b/core/Cargo.toml
@@ -10,6 +10,9 @@ homepage.workspace = true
 categories.workspace = true
 keywords.workspace = true
 
+[features]
+auto-detect-theme = ["dep:dark-light"]
+
 [dependencies]
 bitflags.workspace = true
 glam.workspace = true
@@ -22,6 +25,9 @@ thiserror.workspace = true
 web-time.workspace = true
 xxhash-rust.workspace = true
 
+dark-light.workspace = true
+dark-light.optional = true
+
 [target.'cfg(windows)'.dependencies]
 raw-window-handle.workspace = true
 
diff --git a/core/src/theme.rs b/core/src/theme.rs
index 948aaf83..6b2c04da 100644
--- a/core/src/theme.rs
+++ b/core/src/theme.rs
@@ -7,10 +7,9 @@ use std::fmt;
 use std::sync::Arc;
 
 /// A built-in theme.
-#[derive(Debug, Clone, PartialEq, Default)]
+#[derive(Debug, Clone, PartialEq)]
 pub enum Theme {
     /// The built-in light variant.
-    #[default]
     Light,
     /// The built-in dark variant.
     Dark,
@@ -161,6 +160,28 @@ impl Theme {
     }
 }
 
+impl Default for Theme {
+    fn default() -> Self {
+        #[cfg(feature = "auto-detect-theme")]
+        {
+            use once_cell::sync::Lazy;
+
+            static DEFAULT: Lazy<Theme> =
+                Lazy::new(|| match dark_light::detect() {
+                    dark_light::Mode::Dark => Theme::Dark,
+                    dark_light::Mode::Light | dark_light::Mode::Default => {
+                        Theme::Light
+                    }
+                });
+
+            DEFAULT.clone()
+        }
+
+        #[cfg(not(feature = "auto-detect-theme"))]
+        Theme::Light
+    }
+}
+
 impl fmt::Display for Theme {
     fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
         match self {