update to kotlin 1.3.40, added theme choose option

* added missing dividers in settings
* added a black theme
* closes #15

build.gradle

@@ -1,7 +1,7 @@
 // Top-level build file where you can add configuration options common to all sub-projects/modules.
 
 buildscript {
-    ext.kotlin_version = '1.3.31'
+    ext.kotlin_version = '1.3.40'
     repositories {
         google()
         jcenter()