Added dark theme

app/src/main/java/org/schabi/newpipe/Themer.java

@@ -0,0 +1,20 @@
+package org.schabi.newpipe;
+
+import android.os.Bundle;
+import android.preference.PreferenceManager;
+import android.support.annotation.Nullable;
+import android.support.v7.app.AppCompatActivity;
+
+import java.util.Objects;
+
+public class Themer extends AppCompatActivity {
+
+    @Override
+    protected void onCreate(@Nullable Bundle savedInstanceState) {
+        super.onCreate(savedInstanceState);
+        if (Objects.equals(PreferenceManager.getDefaultSharedPreferences(this)
+                .getString("theme", "1"), "0")) {
+            setTheme(R.style.DarkTheme);
+        }
+    }
+}