don't change bg color on ios

src/alf/util/useColorModeTheme.ts

@@ -1,11 +1,11 @@
 import React from 'react'
 import {ColorSchemeName, useColorScheme} from 'react-native'
-
-import {useThemePrefs} from 'state/shell'
-import {isWeb} from 'platform/detection'
-import {ThemeName, light, dark, dim} from '#/alf/themes'
 import * as SystemUI from 'expo-system-ui'
 
+import {isIOS, isWeb} from 'platform/detection'
+import {useThemePrefs} from 'state/shell'
+import {dark, dim, light, ThemeName} from '#/alf/themes'
+
 export function useColorModeTheme(): ThemeName {
   const colorScheme = useColorScheme()
   const {colorMode, darkTheme} = useThemePrefs()
@@ -13,7 +13,11 @@ export function useColorModeTheme(): ThemeName {
   React.useLayoutEffect(() => {
     const theme = getThemeName(colorScheme, colorMode, darkTheme)
     updateDocument(theme)
-    SystemUI.setBackgroundColorAsync(getBackgroundColor(theme))
+
+    // must be default on iOS due to the formSheet modal presentation style
+    if (!isIOS) {
+      SystemUI.setBackgroundColorAsync(getBackgroundColor(theme))
+    }
   }, [colorMode, colorScheme, darkTheme])
 
   return React.useMemo(