Going by the roadmap, dark mode isn't even under consideration. Considering how many responses this has gotten, it seems like maybe this should be a priority. Comparatively few people visit these forums and even fewer comment, so (currently) 83 requests for this is statistically pretty high.
Also noteworthy is that many accessibility features in general (dark mode is considered an accessibility feature, by the way) are not prioritized. "Closed Captions For Video" and "Accessible Feedback Layers" are currently in development, which is good, but "Video Transcripts," "Multiple Audio Tracks," and "Multiple Closed Captions" are below a number of "nice to have" features. IMO you should be prioritizing ALL accessibility features. Being fully accessible is something that will improve usability for countless users of both Storyline and the projects we create with it.