How do you fix androidmanifest coding

My code is
<?xml version="1.0" encoding="utf-8"?>
<manifest xmlns:android="http://schemas.android.com/apk/res/android"
package="com.Second app">

<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/AppTheme">
<activity android:name=".MainActivity">
<intent-filter>
<action android:name="android.intent.action.MAIN" />
<category android:name="android.intent.category.LAUNCHER" />
</intent-filter>
</activity>
</application>
</manifest>

The error are
Cannot resolve symbol
"@Style/AppTheme">
Unresolved class "MainActivity"