fenix/app/src/main/res/drawable/progress_gradient.xml

<?xml version="1.0" encoding="utf-8"?>
<!-- This Source Code Form is subject to the terms of the Mozilla Public
   - License, v. 2.0. If a copy of the MPL was not distributed with this
   - file, You can obtain one at http://mozilla.org/MPL/2.0/. -->
<layer-list xmlns:android="http://schemas.android.com/apk/res/android">
    <item android:id="@android:id/background">
        <shape>
            <solid android:color="?above" />
        </shape>
    </item>

    <item android:id="@android:id/progress">
        <scale android:scaleWidth="100%">
            <shape>
                <corners android:radius="8dp" />
                <gradient
                    android:angle="45"
                    android:centerColor="#F10366"
                    android:endColor="#FF9100"
                    android:startColor="#6173FF" />
            </shape>
        </scale>
    </item>
</layer-list>