theme selection & gradle update

* it's now possible to change the app theme (light/dark)
* update gradle to version 6.7.1
* update gradle pugin to version 4.1.1
* update kotlin to  1.4.10

app/src/main/java/org/mosad/teapod/util/DataTypes.kt

@@ -6,6 +6,11 @@ class DataTypes {
         MOVIE,
         TVSHOW
     }
+
+    enum class Theme(val str: String) {
+        LIGHT("Light"),
+        DARK("Dark")
+    }
 }
 
 /**