Web & UI: resolve system theme preference and default web to system theme

apps/gridcraft-web/src/web.rs

@@ -38,6 +38,7 @@ pub fn start() {
                         let _ = session.execute("file.new", serde_json::json!({"sample": "sales"}));
                     }
                     let mut app = SheetApp::new(session, services(inbox.clone(), cc.egui_ctx.clone()));
+                    app.ui.system_theme = true;
                     // Follow the browser's language, except a CJK one: the web build has no system
                     // CJK fonts (none are bundled), so that interface would render as tofu.
                     let browser = gridcraft_ui_egui::i18n::Language::system();

contributors/people.toml

@@ -45,3 +45,7 @@ sync_github_name = false
 
 [people.Losminek]
 sync_github_name = true
+
+[people.GhostMitra]
+sync_github_name = true
+

crates/ui-egui/src/theme.rs

@@ -125,6 +125,21 @@ impl Tokens {
     }
 }
 
+/// Resolves an [`egui::ThemePreference`] against the context's system theme.
+pub fn resolve_theme(pref: egui::ThemePreference, ctx: &egui::Context) -> egui::Theme {
+    match pref {
+        egui::ThemePreference::System => {
+            if ctx.input(|i| i.raw.system_theme == Some(egui::Theme::Dark)) {
+                egui::Theme::Dark
+            } else {
+                egui::Theme::Light
+            }
+        }
+        egui::ThemePreference::Light => egui::Theme::Light,
+        egui::ThemePreference::Dark => egui::Theme::Dark,
+    }
+}
+
 pub const UI: &str = "ui";
 pub const UI_BOLD: &str = "ui-bold";
 pub const CELL: &str = "cell";

crates/ui-egui/src/widgets.rs

@@ -236,3 +236,31 @@ pub fn toast(app: &mut SheetApp, ctx: &egui::Context) {
     );
     ctx.request_repaint_after(std::time::Duration::from_millis(200));
 }
+
+/// Turn a monochrome image (black on white) into an alpha mask tinted with the active text color.
+pub fn raster_to_alpha_mask(pixels: &mut [egui::Color32]) {
+    for p in pixels.iter_mut() {
+        // Darkness of source becomes the opacity
+        let darkness = 255u8.saturating_sub(p.r());
+        // Pure white glyph with variable alpha
+        *p = egui::Color32::from_rgba_unmultiplied(255, 255, 255, darkness);
+    }
+}
+
+#[cfg(test)]
+mod tests {
+    use super::*;
+
+    #[test]
+    fn test_raster_to_alpha_mask() {
+        let mut pixels = vec![
+            egui::Color32::WHITE,                   // r=255 -> darkness=0 -> alpha=0
+            egui::Color32::BLACK,                   // r=0 -> darkness=255 -> alpha=255
+            egui::Color32::from_rgb(100, 100, 100), // r=100 -> darkness=155 -> alpha=155
+        ];
+        raster_to_alpha_mask(&mut pixels);
+        assert_eq!(pixels[0], egui::Color32::from_rgba_unmultiplied(255, 255, 255, 0));
+        assert_eq!(pixels[1], egui::Color32::from_rgba_unmultiplied(255, 255, 255, 255));
+        assert_eq!(pixels[2], egui::Color32::from_rgba_unmultiplied(255, 255, 255, 155));
+    }
+}