diff --git a/src/renderer/context/App.tsx b/src/renderer/context/App.tsx
index 4ca7a5f40..14b085815 100644
--- a/src/renderer/context/App.tsx
+++ b/src/renderer/context/App.tsx
@@ -55,7 +55,9 @@ import {
 import { clearState, loadState, saveState } from '../utils/storage';
 import {
   DEFAULT_DAY_COLOR_SCHEME,
+  DEFAULT_DAY_HIGH_CONTRAST_COLOR_SCHEME,
   DEFAULT_NIGHT_COLOR_SCHEME,
+  DEFAULT_NIGHT_HIGH_CONTRAST_COLOR_SCHEME,
   mapThemeModeToColorMode,
   mapThemeModeToColorScheme,
 } from '../utils/theme';
@@ -276,8 +278,18 @@ export const AppProvider = ({ children }: { children: ReactNode }) => {
     );
 
     setColorMode(colorMode);
-    setDayScheme(colorScheme ?? DEFAULT_DAY_COLOR_SCHEME);
-    setNightScheme(colorScheme ?? DEFAULT_NIGHT_COLOR_SCHEME);
+
+    // When colorScheme is null (System theme), use appropriate fallbacks
+    // based on whether high contrast is enabled
+    const dayFallback = settings.increaseContrast
+      ? DEFAULT_DAY_HIGH_CONTRAST_COLOR_SCHEME
+      : DEFAULT_DAY_COLOR_SCHEME;
+    const nightFallback = settings.increaseContrast
+      ? DEFAULT_NIGHT_HIGH_CONTRAST_COLOR_SCHEME
+      : DEFAULT_NIGHT_COLOR_SCHEME;
+
+    setDayScheme(colorScheme ?? dayFallback);
+    setNightScheme(colorScheme ?? nightFallback);
   }, [
     settings.theme,
     settings.increaseContrast,
diff --git a/src/renderer/utils/theme.test.ts b/src/renderer/utils/theme.test.ts
index 9701dc293..6f176c39f 100644
--- a/src/renderer/utils/theme.test.ts
+++ b/src/renderer/utils/theme.test.ts
@@ -1,5 +1,12 @@
 import { Theme } from '../types';
-import { mapThemeModeToColorMode, mapThemeModeToColorScheme } from './theme';
+import {
+  DEFAULT_DAY_COLOR_SCHEME,
+  DEFAULT_DAY_HIGH_CONTRAST_COLOR_SCHEME,
+  DEFAULT_NIGHT_COLOR_SCHEME,
+  DEFAULT_NIGHT_HIGH_CONTRAST_COLOR_SCHEME,
+  mapThemeModeToColorMode,
+  mapThemeModeToColorScheme,
+} from './theme';
 
 describe('renderer/utils/theme.ts', () => {
   it('should map theme mode to github primer color mode', () => {
@@ -53,5 +60,16 @@ describe('renderer/utils/theme.ts', () => {
       'dark_dimmed_high_contrast',
     );
     expect(mapThemeModeToColorScheme(Theme.SYSTEM, false)).toBe(null);
+    expect(mapThemeModeToColorScheme(Theme.SYSTEM, true)).toBe(null);
+  });
+
+  it('should export high contrast color scheme constants', () => {
+    expect(DEFAULT_DAY_HIGH_CONTRAST_COLOR_SCHEME).toBe('light_high_contrast');
+    expect(DEFAULT_NIGHT_HIGH_CONTRAST_COLOR_SCHEME).toBe('dark_high_contrast');
+  });
+
+  it('should export default color scheme constants', () => {
+    expect(DEFAULT_DAY_COLOR_SCHEME).toBe('light');
+    expect(DEFAULT_NIGHT_COLOR_SCHEME).toBe('dark');
   });
 });
diff --git a/src/renderer/utils/theme.ts b/src/renderer/utils/theme.ts
index 8a3ba4976..f0131b80c 100644
--- a/src/renderer/utils/theme.ts
+++ b/src/renderer/utils/theme.ts
@@ -4,6 +4,8 @@ import { Theme } from '../types';
 
 export const DEFAULT_DAY_COLOR_SCHEME = 'light';
 export const DEFAULT_NIGHT_COLOR_SCHEME = 'dark';
+export const DEFAULT_DAY_HIGH_CONTRAST_COLOR_SCHEME = 'light_high_contrast';
+export const DEFAULT_NIGHT_HIGH_CONTRAST_COLOR_SCHEME = 'dark_high_contrast';
 
 export function mapThemeModeToColorMode(themeMode: Theme): ColorModeWithAuto {
   switch (themeMode) {