tachiyomi/app/src/main/res/drawable/action_mode_bg.xml

<?xml version="1.0" encoding="utf-8"?>
<layer-list xmlns:android="http://schemas.android.com/apk/res/android">
    <item android:bottom="5dp">
        <shape>
            <solid android:color="?attr/colorPrimaryVariant"/>
        </shape>
    </item>
    <item android:height="5dp" android:gravity="bottom">
        <shape>
            <gradient
                android:angle="90"
                android:endColor="@color/actionModeShadow"
                android:startColor="@android:color/transparent"
                android:type="linear" />
        </shape>
    </item>
</layer-list>