riff_app/app/src/main/res/drawable/seek_bar_progress.xml

<?xml version="1.0" encoding="utf-8"?>
<!-- shamelessly ripped off https://stackoverflow.com/questions/32212259/android-how-to-change-default-seekbar-thickness -->
<layer-list xmlns:android="http://schemas.android.com/apk/res/android">

    <item android:id="@android:id/background">
        <shape>

            <corners
                android:radius="4dp"
                />
            <solid
                android:color="@color/seek_bar_background"
                />
        </shape>
    </item>

    <item android:id="@android:id/secondaryProgress">
        <clip>
            <shape>
                <corners
                    android:radius="4dp"
                    />
                <solid
                    android:color="@color/seek_bar_secondary_progress"
                    />
            </shape>
        </clip>
    </item>

    <item android:id="@android:id/progress">
        <clip>
            <shape>
                <corners
                    android:radius="4dp"
                    />
                <solid
                    android:color="@color/seek_bar_progress"
                    />
            </shape>
        </clip>
    </item>
</layer-list>