The answer is not so simple and whilst the question should be focussed on pure usabiity, we cannot get away from the fact dark modes are very much a trend at the moment.
Some interfaces are designed with brand and trend in mind, usually aligned with whether the platform looks ‘fresh’. The most succesful platforms however, are designed with the end-user in mind, concentrating on critical factors: like whether or not it will improve usability.
Readability: even diametric opposites such as black-on-white or white-on-black have their pros and cons. For most cases, black text on a light UI remains the most readable. However, while this is the most readable, it also can casue fatigue tfor long periods of time. That is why text-heavy interfaces where the user will be reading for extended periods of time often opt for a dark background with light text. It is still readable, but it will not strain the eyes as much as its inverse.
User Environment: this refers to both where and when the user is interfacing with the platform. The ‘when’ can be simplified as day or night, dark interfaces are ideal for nighttime or evening environments, whereas lighter ones are suited for the daytime. The harsh bright light of a smartphone can cause discomfort when viewed in a dark room, such as a plant room for instance and the muted aesthetic of a dark UI will not show up as well in a brightly lit area.
