`egui_kittest`: Add `HarnessBuilder::theme` (#7289)

Makes it ergonomic to snapshot test light vs dark mode

crates/egui_kittest/src/builder.rs

@@ -7,6 +7,7 @@ use std::marker::PhantomData;
 pub struct HarnessBuilder<State = ()> {
     pub(crate) screen_rect: Rect,
     pub(crate) pixels_per_point: f32,
+    pub(crate) theme: egui::Theme,
     pub(crate) max_steps: u64,
     pub(crate) step_dt: f32,
     pub(crate) state: PhantomData<State>,
@@ -19,6 +20,7 @@ impl<State> Default for HarnessBuilder<State> {
         Self {
             screen_rect: Rect::from_min_size(Pos2::ZERO, Vec2::new(800.0, 600.0)),
             pixels_per_point: 1.0,
+            theme: egui::Theme::Dark,
             state: PhantomData,
             renderer: Box::new(LazyRenderer::default()),
             max_steps: 4,
@@ -45,6 +47,13 @@ impl<State> HarnessBuilder<State> {
         self
     }
 
+    /// Set the desired theme (dark or light).
+    #[inline]
+    pub fn with_theme(mut self, theme: egui::Theme) -> Self {
+        self.theme = theme;
+        self
+    }
+
     /// Set the maximum number of steps to run when calling [`Harness::run`].
     ///
     /// Default is 4.

crates/egui_kittest/src/lib.rs

@@ -86,6 +86,7 @@ impl<'a, State> Harness<'a, State> {
         let HarnessBuilder {
             screen_rect,
             pixels_per_point,
+            theme,
             max_steps,
             step_dt,
             state: _,
@@ -93,6 +94,7 @@ impl<'a, State> Harness<'a, State> {
             wait_for_pending_images,
         } = builder;
         let ctx = ctx.unwrap_or_default();
+        ctx.set_theme(theme);
         ctx.enable_accesskit();
         // Disable cursor blinking so it doesn't interfere with snapshots
         ctx.all_styles_mut(|style| style.visuals.text_cursor.blink = false);