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

build.gradle

@@ -1,12 +1,12 @@
 // Top-level build file where you can add configuration options common to all sub-projects/modules.
 buildscript {
-    ext.kotlin_version = "1.4.10"
+    ext.kotlin_version = "1.4.20"
     repositories {
         google()
         jcenter()
     }
     dependencies {
-        classpath 'com.android.tools.build:gradle:4.1.0'
+        classpath 'com.android.tools.build:gradle:4.1.1'
         classpath "org.jetbrains.kotlin:kotlin-gradle-plugin:$kotlin_version"
 
         // NOTE: Do not place your application dependencies here; they belong