1：新建一个项目运行起来，可以看到顶部一直有个标题栏看着不是很美观

2：有两种方法可以去除顶部标题栏
（1）将代码中AndroidManifest里android:theme="@style/Theme.MyApplication"改为android:theme="@style/Theme.AppCompat.NoActionBar">

<application
        android:allowBackup="true"
        android:icon="@mipmap/ic_launcher"
        android:label="@string/app_name"
        android:roundIcon="@mipmap/ic_launcher_round"
        android:supportsRtl="true"
        android:theme="@style/Theme.AppCompat.NoActionBar"

        >

（2）在themes.xml文件里改，选中MyApplication，Ctrl+鼠标点击

来到themes.xml文件里，把代码中parent="Theme.MaterialComponents.DayNight.DarkActionBar"替换为
parent=“Theme.AppCompat.Light.NoActionBar”

 <style name="Theme.MyApplication" parent="Theme.MaterialComponents.DayNight.DarkActionBar">
        <!-- Primary brand color. -->
        <item name="colorPrimary">@color/purple_500</item>
        <item name="colorPrimaryVariant">@color/purple_700</item>
        <item name="colorOnPrimary">@color/white</item>
        <!-- Secondary brand color. -->
        <item name="colorSecondary">@color/teal_200</item>
        <item name="colorSecondaryVariant">@color/teal_700</item>
        <item name="colorOnSecondary">@color/black</item>
        <!-- Status bar color. -->
        <item name="android:statusBarColor" tools:targetApi="l">?attr/colorPrimaryVariant</item>
        <!-- Customize your theme here. -->
    </style>

3：效果展示