First things first, if you haven't already, you'll need to add Firebase to your Android project. Just follow the Firebase guide for adding it to your app. You'll need to create a Firebase account, add a Google services config file, and integrate the Firebase SDK into your project. It's pretty straightforward, I promise! Alright, now that Firebase is in place, let's add Firebase Remote Config to your app. You'll need to import FirebaseRemoteConfig in your app-level build.gradle file.

implementation 'com.google.firebase:firebase-config:19.2.0'

In your main activity, or wherever you want to fetch the theme color, get an instance of Firebase Remote Config like this:

FirebaseRemoteConfig mFirebaseRemoteConfig = FirebaseRemoteConfig.getInstance();

Next up, we need to set some default values for Firebase Remote Config. This is super important because it ensures that values load immediately when the app opens for the first time. Sometimes remote values can take a few seconds to fetch, depending on your internet speed.

Create an XML resource file and define your default values:

<defaultsMap>
    <entry>
        <key>theme_color</key>
        <value>#000000</value>
    </entry>
</defaultsMap>

Load these default values into Firebase Remote Config:

FirebaseRemoteConfig .setDefaultsAsync(R.xml.remote_config_defaults);

Now, let's fetch these values from the Firebase Console. Fetch and activate them with the following code:

mFirebaseRemoteConfig .fetchAndActivate()
  .addOnCompleteListener(task -> {
      if (task.isSuccessful()) {
          boolean updated = task.getResult();
          Log.d(TAG, "Config params updated: " + updated);
      } else {
          ...
      }
}); 

Once the activation is successful, apply these values in your app to change the theme color:

String themeColor = mFirebaseRemoteConfig.getString("theme_color");
getWindow().getDecorView().setBackgroundColor(Color.parseColor(themeColor));

Now that our app is ready to handle theme color dynamically, we can change the theme color of the app from Firebase Console. Head over to the Firebase console, click on Remote Config under Grow. Here, you can add a new parameter with a key of "theme\_color" and a value of any color you want. Click on Publish changes.

Once changes are published, reopen your app and you will see the newly set theme color.

Keep in mind that remote config data is cached and the default cache time is 12 hours, which means the config data will be fetched from Firebase every 12 hours. You can change this cache time while in the development phase.

To change the cache time, set the cache expiration to a lower value before calling fetch:

mFirebaseRemoteConfig.fetch(0);

The above code will cause the fetch call to bypass the cache and directly fetch the data from Firebase.