Answer: gnobal
(Best answer)
Add the following styles to your res/values/styles.xml file (create one if you don’t have one yet). This is a complete file:

<?xml version="1.0" encoding="utf-8"?>
<resources>
  <style name="Theme.Transparent" parent="android:Theme">
    <item name="android:windowIsTranslucent">true</item>
    <item name="android:windowBackground">@android:color/transparent</item>
    <item name="android:windowContentOverlay">@null</item>
    <item name="android:windowNoTitle">true</item>
    <item name="android:windowIsFloating">true</item>
    <item name="android:backgroundDimEnabled">false</item>
  </style>
</resources>

(The color value of @color/transparent I put in the res/values/color.xml file is #00000000. You can also use @android:color/transparent in later android versions)
Then apply the style to your activity, for example:

<activity android:name=".SampleActivity" android:theme="@style/Theme.Transparent">
...
</activity>

Answer: yanchenko
It can look like this:

<activity android:name=".usual.activity.Declaration" android:theme="@android:style/Theme.Translucent.NoTitleBar" />

Answer: Deepak Swami
You can use the following method to set it in the key list

<activity android:name=".yourActivity" android:theme="@android:style/Theme.Translucent.NoTitleBar.Fullscreen"/>

Then add a transparent background to the layout.

Answer: androabhay
In the key list, set the activity tag by adding the theme "@android:style/Theme.Translucent".

Answer: jian
Just set the activity background image to be transparent, or add a theme in the xml file,

<activity android:name=".usual.activity.Declaration" android:theme="@android:style/Theme.Translucent.NoTitleBar" />