For some time now, Mozilla's mobile browser has had a "Night Mode" option, which inverts the colors of web pages except for images and certain other elements, similar to the way Apple's Smart Invert works for the iOS interface.
Version 13 of the app, released today, adds a new Dark display theme that essentially augments the Night Mode by darkening the interface. Used in tandem, Firefox offers users probably the best night-time browsing experience currently available on iOS.
To activate the night-time options, tap the Menu button (the three-line icon at the lower right of the interface) and enable the Night Mode using the toggle button. Then select Settings -> Display, and choose the Dark theme.
In addition to the above, version 13 of Firefox adds a couple of useful functions for users who tend to have a lot of tabs open at the same time. There's now a search bar in the open tabs screen to help you find tabs containing specific content, and individual tabs can now be dragged to rearrange them.
Firefox web browser is a free download for iPhone and iPad available on the App Store. [Direct Link]