Anadi Login al stack navigator en App.js. Quitamos el <View> de Home_page.js para que se vea completo el background.

App.js

       return (
         <NavigationContainer>
           <Stack.Navigator>
-            <Stack.Screen name={" "} options={{headerShown: false}} component={LoginScreen}/>
+            <Stack.Screen name={"Login"} options={{headerShown: false}} component={LoginScreen}/>
             <Stack.Screen name={"Register"} options={{headerTransparent: true, headerTitle: " "}} component={RegisterScreen}/>
           </Stack.Navigator>
         </NavigationContainer>

screens/main/Home_page.js

   const [threads, setThreads] = useState([]);  
   const [loading, setLoading] = useState(true);
   
-  //const [roomName, setRoomName] = useState('');
+  const [roomName, setRoomName] = useState('');
   //const navigation = useNavigation();
+  //const [roomName, setRoomName] = useState('');
 
   useEffect(() => {
 
   }
 
 
-  /*function handleButtonPress() {
+  function handleButtonPress() {
     if (roomName.length > 0) {
       firebase.firestore()
         .collection('Users')
         .add({
           name: roomName
         })
-        .then(() => {
-          navigation.navigate('allChats');
-         });
+        //.then(() => {
+          //navigation.navigate('allChats');
+         //});
     }
-  }*/
+  }
 
 
 
 
   
   return (
-    <View>
     <ImageBackground style={styles.stdcontainer} source={require('../../assets/yellow-white.jpg')}>
         <FlatList
           data={threads}
         )}
       />
 
+                <TextInput
+                    labelName='Room Name'
+                    placeholder="new chat room"
+                    onChangeText={(text) => setRoomName(text)}
+                    clearButtonMode='while-editing'
+                />
+
       <Button
-        title='Ver mensajes'
-        onPress={() => navigation.navigate('Add')}
+        title='Provisional'
+        onPress={() => handleButtonPress()}
       />
 
         <Button
         onPress= {() => navigation.navigate('Room')}
         />
         </ImageBackground>
-      </View>
     );
   }