Activate Dark Theme on Calendar. Follow below steps. Step 1, Open calendar app and then click on top-left corner menu button. Step 2, Now menu list open and have Settings options. Click on settings. Step 3, Now new settings page is open and have General option. Click on General. Step 4, Now swipe down and have Theme option and yet selected Light. For change, click on Theme . Step 5, Now, you can see three option. Light, Dark and Set by Battery Saver. So for making dark. Click on dark option. Now, Your app is in dark Theme. Enjoy Dark Theme .