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

<?xml version="1.0" encoding="utf-8"?>
<selector xmlns:android="http://schemas.android.com/apk/res/android">
    <item android:state_selected="true">
        <shape android:shape="rectangle">
            <corners android:radius="8dp" />
            <solid android:color="?attr/colorAccent" />
            <size android:width="6dp" android:height="54dp" />
        </shape>
    </item>
    <item>
        <shape android:shape="rectangle">
            <corners android:radius="8dp" />
            <solid android:color="@color/fast_scroller_handle_idle" />
            <size android:width="6dp" android:height="54dp" />
        </shape>
    </item>
</selector>