PresentationTheme: Bright: Set cellbackgroundA to 0xFFFFFF

According to design

SharedSources/PresentationTheme.swift

@@ -147,7 +147,7 @@ let brightPalette = ColorPalette(isDark: false,
                                  navigationbarColor: UIColor(0xFFFFFF),
                                  navigationbarTextColor: UIColor(0x000000),
                                  background: UIColor(0xFFFFFF),
-                                 cellBackgroundA: UIColor(0xF9F9F7),
+                                 cellBackgroundA: UIColor(0xFFFFFF),
                                  cellBackgroundB: UIColor(0xE5E5E3),
                                  cellDetailTextColor: UIColor(0xA9A9A9),
                                  cellTextColor: UIColor(0x000000),