fix(color-scheme): set color scheme dummy value to null

Set the initial value of the color scheme ref to null to make setting it
to the actual value trigger a change if the color scheme is auto.

src/App.vue

@@ -26,7 +26,7 @@ import { useRoute } from 'vue-router';
 const route = useRoute();
 
 const { getColorScheme, updateColorScheme } = useColorScheme();
-const colorScheme = ref('auto');
+const colorScheme = ref(null);
 provide('colorScheme', colorScheme);
 watch(colorScheme, (newValue) => {
     updateColorScheme(newValue);