Screenshots of "Dark Mode" in iOS 10. Image source: Andrew Wilk via Twitter.

If there remains any doubt that Apple (NASDAQ:AAPL) is working to shift its iPhones away from LCD technology and toward OLED displays, that doubt should be fading fast after a new hint of the tech giant's plans for its most popular product line.

Earlier this week, programmer Andrew Wilk posted screenshots from an iOS 10 beta simulator -- which makes it easier for clever developers to uncover hidden or unfinished methods in the code -- highlighting a yet-to-be-released "Dark Mode" interface for the mobile operating system. 

But what does this have to do with OLED? In short, OLED displays function most effectively with darker backgrounds, from both aesthetic and energy-efficient points of view.

For perspective, recall that late last year Sidney Rosenblatt, CFO of OLED technologist Universal Display (NASDAQ:OLED), caused shares of his company to rise after asserting "it's only a matter of time" before Apple puts OLED displays into its iPhones.

Rosenblatt prefaced that prediction by discussing comments from Apple CTO Jony Ive made in a New Yorker article in early 2015.  Speaking on a conference call, Rosenblatt said:

There is a section in there where he talks about his screen on his iPhone 6 -- which hadn't even come out -- is old technology. It looks old. And [he] talks about a new technology they're going to use for their watches that only puts current to the pixels that light up, gives you a true black background, and gives you a much thinner form factor, and a much more power-efficient device. He talks about it being magic. Essentially, it's an OLED.

Of course, we know now that the Apple Watch does use an OLED display, with the first iterations manufactured and provided by Universal Display customer LG Display. But as it stands, the relatively small screen areas and lower sales volumes of the Apple Watch have resulted in a negligible positive impact to Universal Display's top and bottom lines.

To be fair, a Dark Mode interface on iOS doesn't guarantee Apple is taking the OLED iPhone plunge. After all, Apple already confirmed at its latest Worldwide Developer Conference it would implement Dark Mode for its TVOS on Apple TV, primarily citing the color scheme's value in lower-light home theater environments.

But we should also remember that in addition to Rosenblatt's comments, South Korean news outlets reported in April that fellow UDC customer Samsung Display signed a multi-year deal, worth an estimated $2.59 billion, to supply Apple with 100 million 5.5-inch OLED smartphone displays annually.

Before that in January 2016, Taiwanese news outlets suggested Apple was mulling a large investment in AU Optronics -- another UDC customer -- to "transform" it from primarily an LCD manufacturer into a yet another reliable OLED vendor.

Most recently, shares of Universal Display touched an all-time high earlier this month, after maker of electronics manufacturing equipment Applied Materials credited a record quarter from its Display segment to a massive increase in OLED-centric equipment orders. In a Bloomberg interview later, without specifically naming names, Applied Materials CEO Gary Dickerson strongly insinuated those orders came from Apple.

In the end, combined with Andrew Wilk's discovery of Dark Mode for iOS: if all this evidence isn't enough to convincingly point to Apple's impending transition to OLED iPhones, I don't know what is.

This article represents the opinion of the writer, who may disagree with the “official” recommendation position of a Motley Fool premium advisory service. We’re motley! Questioning an investing thesis -- even one of our own -- helps us all think critically about investing and make decisions that help us become smarter, happier, and richer.