Extend your theme from .Dialog.Alert

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<style name="YOUR_THEME_NAME" parent="@style/Theme.AppCompat.Light.Dialog.Alert">
    <!--your theme here-->
</style>

Do not extend your theme from .Dialog, if you want your dialog to match parent/window’s width.

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<style name="YOUR_THEME_NAME" parent="@style/Theme.AppCompat.Light.Dialog.Alert">
    <!--your theme here-->
</style>