Gmail is getting a dark mode just like many of Google's other Android applications and while it's not fully functional just yet you can catch a glimpse of what it will be like once Google has everything fixed up nice and pretty.
For clarification sake, the dark mode is technically something you can already enable if you have the latest version of the Gmail app and if you have the Night Mode option in developer settings turned on, but it seems like it's not entirely baked and will need a little more time in the oven as the text appears to be visually darker than Google is intending it to be for a full stable rollout.
First things first, before you can enable dark mode in your Gmail app you'll need to make sure that you're on the latest version of Gmail which Google just recently started pushing out yesterday. This will need to be version 2019.06.09, and although some users may have already gotten the update it also may have not shown up for everyone yet.
If you're on the latest version of Gmail, then your next step to turning on dark mode will be to enable the developer options in your phone's settings menu if you haven't already done so. You can find developer option inside the System sub-menu from within settings. You'll need to go in here and scroll until you see the Night Mode option, tap it, and switch it to always on.
If you don't have developer options enabled yet, doing so is easy enough and won't take very much effort. Instead of going into the System sub-menu, scroll from the regular settings menu down to About Phone and then tap on this menu, then scroll all the way to the bottom where you'll find the build number information.
Tapping this rapidly will surface some popup messages that tell you you're about to enable developer options and finally that you're now a developer. From here you can go back to the System menu, tap on advanced options and enter Developer Options to enable Night Mode.
After all of this is completed then all that's left is to go into Gmail and tap on the settings from the navigation menu that slides out from the left of the screen. Once you're here, this first screen is where things will showcase that the dark mode in the Gmail app is not fully finished as you won't be able to read the text very easily.
Once you tap on any of those first menu options though, such as the General Settings, everything gets a lot brighter and is more like what you should expect when the dark mode options is completely ready. Text should be noticeably brighter against the dark background making much simpler to see what it is you're tapping on.
Text for the menus will appear blue as your headers, with the descriptions for each of those having text that's both dark grey and more of a light grey or almost white, making for a much more visible contrast to the background of the menu.
It's also important to remember that this is just for the settings menu inside of Gmail right now and it won't appear in the actual inboxes or the rest of the app. Once all of that happens you may also not need Night Mode to be turned on inside of the Developer Options, and should just be able to open Gmail and see the darker colors. The feature as it currently sits also seems to be a bit unstable and goes back to the regular white menu background from time to time, so closing out the app from recents and then re-opening may fix this.