k-9/res/values-v11/themes.xml

<?xml version="1.0" encoding="utf-8"?>
<resources xmlns:android="http://schemas.android.com/apk/res/android">

    <style name="Theme.K9.Dark" parent="Theme.K9.Dark.Base">
        <item name="composerBackgroundColor">@android:color/background_dark</item>
    </style>

    <style name="Theme.K9.Light" parent="Theme.K9.Light.Base">
        <item name="composerBackgroundColor">@android:color/background_light</item>
    </style>

    <style name="Theme.K9.Dialog.Dark" parent="Theme.K9.Dark">
        <item name="backgroundColorChooseAccountHeader">#404040</item>
    </style>

</resources>