PresentationTheme: Update cellBackgroundB to be a variant of cellBackgroundA

SharedSources/PresentationTheme.swift

@@ -166,7 +166,7 @@ let darkPalette = ColorPalette(isDark: true,
                                navigationbarTextColor: UIColor(0xFFFFFF),
                                background: UIColor(0x1B1E21),
                                cellBackgroundA: UIColor(0x1B1E21),
-                               cellBackgroundB: UIColor(0x000000),
+                               cellBackgroundB: UIColor(0x494B4D),
                                cellDetailTextColor: UIColor(0xD3D3D3),
                                cellTextColor: UIColor(0xFFFFFF),
                                lightTextColor: UIColor(0xB8B8B8),