Custom Alert Dialog looking weird on Android 4.x

I fixed it by using below code:

 <style name="CustomAppCompatAlertDialogStyle" parent="Theme.AppCompat.Light.Dialog.Alert">
    <item name="android:windowBackground">@android:color/transparent</item>
</style>

This happens because you´re using AlertDialog instead of the relatively new AlertDialogCompat. This one brings the same Material Design style dialogs across all platforms and is highly encouraged its use.

To use it, you must be sure that in your Gradle file you have the following line inside your dependencies:

com.android.support:appcompat-v7:24.2.1
(This one is the latest version. Make sure you have all your packages updated by looking into the SDK Manager).

Then in the Activity where you want to show it you must import android.support.v7.app.AlertDialog and use the AlertDialog.Builder exactly as you were using it before.

For those of you using AndroidX you'll want to use the following import:

import androidx.appcompat.app.AlertDialog