I could not manage to find a solution with just xml, that is why I used a picture to solve this, with a little help from this answer:
Created an image of the thumb seekbar_thumb_icon.png
and saved it in different resolutions in res/drawable-*
maps (hdpi/mdpi/xhdpi/xxhdpi).
Specified "android:thumb="@layout/seekbar_thumb" for SeekBar:
<layer-list xmlns:android="http://schemas.android.com/apk/res/android"
android:layout_width="match_parent"
android:layout_height="match_parent">
<item>
<shape>
<size
android:height="30dp"
android:width="30dp" />
<solid android:color="@android:color/transparent" />
</shape>
</item>
<item android:drawable="@drawable/balance_icon_48px"/>
</layer-list>
Other styles:
<SeekBar
android:id="@+id/seekbar"
android:layout_width="match_parent"
android:layout_height="wrap_content"
android:progressDrawable="@layout/seekbar_style"
android:thumb="@layout/seekbar_thumb" />
seekbar_style.xml
<layer-list xmlns:android="http://schemas.android.com/apk/res/android" android:layout_height="match_parent"
android:layout_width="match_parent">
<item android:id="@android:id/background"
android:drawable="@layout/seekbar_background" />
<item android:id="@android:id/progress">
<clip android:drawable="@layout/seekbar_progress" />
</item>
</layer-list>
seekbar_background.xml
<shape xmlns:android="http://schemas.android.com/apk/res/android"
android:layout_width="match_parent"
android:layout_height="match_parent"
android:shape="line">
<stroke android:width="2dp" android:color="#868686"/>
<corners android:radius="1dp" />
</shape>
seekbar_progress.xml
<shape xmlns:android="http://schemas.android.com/apk/res/android"
android:layout_width="match_parent"
android:layout_height="match_parent"
android:shape="line">
<stroke android:width="2dp" android:color="#ffffff"/>
<corners android:radius="1dp" />
</shape>