In recent months most of google applications applications have turned extremely white including the new Gmail. Not that it is great for outdoor use but very uncomfortable when used in low light to at night to check your email right before sleeping.
Additionally, if you want a really dark mode experience including all Samsung apps, Google apps, System wide customized theme and constant updates. This app has you covered through all of it as it covers most of common apps used on your phone.
Once installed you can change the theme of the whole system from different colors as shown below. I personally use black due to AMOLED screen as it creates contrast colors and saves some of amount of battery.
In the install/updates section you will find all the apps that can be turned into Dark Mode. That includes Gmail, Google Drive, Calender and so on. Unfortunately, Facebook does not allow overlay hence it does not work on either Facebook or Facebook Messenger.
Overall, it is great application for only $1.00 with very active developers that update the application usually a few days after the overlayed application are updated due to changes in layout. An example of Gmail overlay is shown below