PresentationTheme: BrightPalette: Update background color

This changes the background color according to Figma

SharedSources/PresentationTheme.swift

@@ -146,7 +146,7 @@ let brightPalette = ColorPalette(isDark: false,
                                  statusBarStyle: .default,
                                  navigationbarColor: UIColor(0xFFFFFF),
                                  navigationbarTextColor: UIColor(0x000000),
-                                 background: UIColor(0xF9F9F7),
+                                 background: UIColor(0xFFFFFF),
                                  cellBackgroundA: UIColor(0xF9F9F7),
                                  cellBackgroundB: UIColor(0xE5E5E3),
                                  cellDetailTextColor: UIColor(0xA9A9A9),