packages/CarSystemUI/res/drawable/car_seekbar_thumb.xml

<?xml version="1.0" encoding="utf-8"?>
<!--
  ~ Copyright (C) 2018 The Android Open Source Project
  ~
  ~ Licensed under the Apache License, Version 2.0 (the "License");
  ~ you may not use this file except in compliance with the License.
  ~ You may obtain a copy of the License at
  ~
  ~      http://www.apache.org/licenses/LICENSE-2.0
  ~
  ~ Unless required by applicable law or agreed to in writing, software
  ~ distributed under the License is distributed on an "AS IS" BASIS,
  ~ WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
  ~ See the License for the specific language governing permissions and
  ~ limitations under the License
  -->

<layer-list xmlns:android="http://schemas.android.com/apk/res/android">
    <item>
        <shape android:shape="oval">
            <padding
                android:bottom="@*android:dimen/car_padding_1"
                android:left="@*android:dimen/car_padding_1"
                android:right="@*android:dimen/car_padding_1"
                android:top="@*android:dimen/car_padding_1"/>
            <solid android:color="@android:color/black"/>
        </shape>
    </item>
    <item>
        <shape android:shape="oval">
            <solid android:color="@*android:color/car_accent"/>
            <size
                android:width="@*android:dimen/car_seekbar_thumb_size"
                android:height="@*android:dimen/car_seekbar_thumb_size"/>
        </shape>
    </item>
</layer-list>