Editing MultiAutocompleteTextView style

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The problem

My material MultiAutoCompleteTextView style doesn't seem to be applied. More precisely, I just want to make the bottom default line of my (material!) widget become yellow and 1px of height. By default, it's gray and perhaps 3px of height, as you can see below.

(this is displayed when my activity's fragment starts, and this is the default material style of this MultiAutoCompleteTextView)

What I have tried

A lot of things! :-)

1. I defined a style in my app's theme to override android:autoCompleteTextViewStyle (NB: android:autoCompleteTextViewStyle doesn't exist)

2. In the override, I tried to a) set colors - background color, color, accent color, colorControlNormal, colorFocusedHighlight, etc. etc. etc. - and b) set a background, defined in a XML file. Nothing worked.

Sources

App's theme

<style name="" parent="Theme.AppCompat.Light.NoActionBar">
        <item name="android:autoCompleteTextViewStyle">@style/my_autoCompleteTextViewStyle</item>
    </style>

@style/my_autoCompleteTextViewStyle

<style name="my_autoCompleteTextViewStyle" parent="android:Widget.Material.Light.AutoCompleteTextView">
    <item name="android:background">@drawable/background_autocompletetextview</item>

    <item name="android:color">@color/colorRichYellow</item>
    <item name="android:borderlessButtonStyle">@color/colorRichYellow</item>
    <item name="android:colorAccent">@color/colorRichYellow</item>
    <item name="android:colorControlHighlight">@color/colorRichYellow</item>
    <item name="android:colorActivatedHighlight">@color/colorRichYellow</item>
    <item name="android:colorControlActivated">@color/colorRichYellow</item>
    <item name="android:colorControlNormal">@color/colorRichYellow</item>
    <item name="android:colorBackground">@color/colorRichYellow</item>
    <item name="android:colorFocusedHighlight">@color/colorRichYellow</item>
</style>

@drawable/background_autocompletetextview

<?xml version="1.0" encoding="utf-8"?>
<shape xmlns:android="http://schemas.android.com/apk/res/android" android:shape="line">
    <stroke android:width="1dp" android:color="@color/colorRichYellow" />
</shape>

My question

I don't understand why it doesn't work. I already did that for a spinner (select box), and it perfectly worked... How could I make it work for my material MultiAutoCompleteTextView?

from Editing MultiAutocompleteTextView style