diff --git a/crates/ui-egui/src/theme.rs b/crates/ui-egui/src/theme.rs
index e6c06e5dd..3b1a0524d 100644
--- a/crates/ui-egui/src/theme.rs
+++ b/crates/ui-egui/src/theme.rs
@@ -587,6 +587,15 @@ pub fn apply(ctx: &egui::Context, b: Brightness) {
 pub fn apply_with(ctx: &egui::Context, b: Brightness, accent: Accent, scheme: Scheme) {
     let t = Tokens::for_scheme(scheme, b, accent);
     ctx.data_mut(|d| d.insert_temp(egui::Id::NULL, t));
+    // egui keeps a style for each system theme and by default follows the system's setting. The
+    // chosen brightness sets egui's theme, so these colors stay in force when the system's setting
+    // differs or changes. A change of theme starts from the current style, which keeps settings
+    // such as the tool tip delay.
+    let theme = if t.dark { egui::Theme::Dark } else { egui::Theme::Light };
+    if ctx.theme() != theme {
+        ctx.set_style_of(theme, ctx.global_style());
+    }
+    ctx.set_theme(theme);
     let mut v = if t.dark { Visuals::dark() } else { Visuals::light() };
     v.panel_fill = t.panel;
     v.window_fill = t.panel;
@@ -669,4 +678,55 @@ mod tests {
             assert_eq!(Scheme::parse(Scheme::Classic.id()), Some(Scheme::Classic));
         }
     }
+
+    /// #3818: the brightness chosen in Preferences stays in force when the system's light or dark
+    /// setting differs from it or changes while the app runs. Tool tips keep the theme's panel fill
+    /// and text color instead of egui's own white box and gray text.
+    #[test]
+    fn the_theme_holds_whatever_the_system_theme() {
+        let screen = egui::Rect::from_min_size(egui::Pos2::ZERO, egui::vec2(400.0, 400.0));
+        for b in Brightness::ALL {
+            for scheme in Scheme::ALL {
+                for (first, then) in [(egui::Theme::Dark, egui::Theme::Light), (egui::Theme::Light, egui::Theme::Dark)] {
+                    let t = Tokens::for_scheme(scheme, b, Accent::default());
+                    let ctx = egui::Context::default();
+                    let mut time = 0.0;
+                    let mut tip = None;
+                    for i in 0..5 {
+                        time += 0.5;
+                        let system_theme = Some(if i == 0 { first } else { then });
+                        let events = if i == 1 { vec![egui::Event::PointerMoved(egui::pos2(30.0, 15.0))] } else { vec![] };
+                        let input = egui::RawInput { time: Some(time), screen_rect: Some(screen), events, system_theme, ..Default::default() };
+                        let mut out = ctx.run_ui(input, |ui| {
+                            if i == 0 {
+                                apply_with(ui.ctx(), b, Accent::default(), scheme);
+                                ui.ctx().global_style_mut(|s| s.interaction.tooltip_delay = 0.0);
+                            }
+                            ui.button("Hover me").on_hover_text("Tip text");
+                        });
+                        out.textures_delta.clear();
+                        tip = out.shapes.iter().find_map(|c| match &c.shape {
+                            egui::Shape::Text(s) if s.galley.text() == "Tip text" => Some(s.fallback_color),
+                            _ => None,
+                        });
+                    }
+                    let v = ctx.global_style().visuals.clone();
+                    assert_eq!((v.window_fill, v.override_text_color), (t.panel, Some(t.text)), "{scheme:?} {b:?}, system {first:?} then {then:?}");
+                    assert_eq!(tip, Some(t.text), "{scheme:?} {b:?}, system {first:?} then {then:?}: the tool tip's text");
+                }
+            }
+        }
+    }
+
+    /// Changing brightness keeps the interface settings made after the theme was applied, such as
+    /// General › Show Tool Tips' delay.
+    #[test]
+    fn changing_brightness_keeps_other_interface_settings() {
+        let ctx = egui::Context::default();
+        apply(&ctx, Brightness::Dark);
+        ctx.global_style_mut(|s| s.interaction.tooltip_delay = f32::INFINITY);
+        apply(&ctx, Brightness::Light);
+        assert_eq!(ctx.global_style().interaction.tooltip_delay, f32::INFINITY);
+        assert_eq!(ctx.global_style().visuals.window_fill, Tokens::for_brightness(Brightness::Light).panel);
+    }
 }