Android/feature/src/main/res/drawable/side_nav_bar.xml

<shape xmlns:android="http://schemas.android.com/apk/res/android"
    android:shape="rectangle">
    <gradient
        android:angle="0"
        android:centerColor="#666699"
        android:endColor="#666699"
        android:startColor="#666699"
        android:type="linear" />
</shape>