Add light-mode Widget Gallery screenshot test (#7288)

Part of some work to improve text rendering in light mode (again!)

crates/egui_demo_lib/src/demo/widget_gallery.rs

@@ -319,16 +319,27 @@ mod tests {
             date: Some(chrono::NaiveDate::from_ymd_opt(2024, 1, 1).unwrap()),
             ..Default::default()
         };
-        let mut harness = Harness::builder()
-            .with_pixels_per_point(2.0)
-            .with_size(Vec2::new(380.0, 550.0))
-            .build_ui(|ui| {
-                egui_extras::install_image_loaders(ui.ctx());
-                demo.ui(ui);
-            });
 
-        harness.fit_contents();
+        for pixels_per_point in [1, 2] {
+            for theme in [egui::Theme::Light, egui::Theme::Dark] {
+                let mut harness = Harness::builder()
+                    .with_pixels_per_point(pixels_per_point as f32)
+                    .with_theme(theme)
+                    .with_size(Vec2::new(380.0, 550.0))
+                    .build_ui(|ui| {
+                        egui_extras::install_image_loaders(ui.ctx());
+                        demo.ui(ui);
+                    });
 
-        harness.snapshot("widget_gallery");
+                harness.fit_contents();
+
+                let theme_name = match theme {
+                    egui::Theme::Light => "light",
+                    egui::Theme::Dark => "dark",
+                };
+                let image_name = format!("widget_gallery_{theme_name}_x{pixels_per_point}");
+                harness.snapshot(&image_name);
+            }
+        }
     }
 }

crates/egui_demo_lib/tests/snapshots/widget_gallery_dark_x1.png

@@ -0,0 +1,3 @@
+version https://git-lfs.github.com/spec/v1
+oid sha256:62df72fd7e2404c4aa482f09eff5103ee28e8afc42ee8c8c74307a246f64cda6
+size 64651

crates/egui_demo_lib/tests/snapshots/widget_gallery_light_x1.png

@@ -0,0 +1,3 @@
+version https://git-lfs.github.com/spec/v1
+oid sha256:2198a523fb986e90fa3a42f047499f5b1c791075e7c3822b45509d9880073966
+size 60272

crates/egui_demo_lib/tests/snapshots/widget_gallery_light_x2.png

@@ -0,0 +1,3 @@
+version https://git-lfs.github.com/spec/v1
+oid sha256:7bb371a477f58c90ac72aed45a081f3177ea968f090e3739bdb5044ade29f4be
+size 144295