fix: respond to media query change events when theme is auto (#1731)

Co-authored-by: Chris Swithinbank <swithinbank@gmail.com>

.changeset/swift-files-wave.md

@@ -0,0 +1,5 @@
+---
+'@astrojs/starlight': patch
+---
+
+Fixes responding to system color scheme changes when theme is `auto`

packages/starlight/components/ThemeSelect.astro

@@ -42,6 +42,9 @@ const { labels } = Astro.props;
 						this.#onThemeChange(this.#parseTheme(e.currentTarget.value));
 					}
 				});
+				matchMedia(`(prefers-color-scheme: light)`).addEventListener('change', () => {
+					if (this.#loadTheme() === 'auto') this.#onThemeChange('auto');
+				});
 			}
 		}