Fluke handheld multimeters have been dark for decades. Tektronix famously used (and still uses) turquoise. Keithley used milk chocolate brown for years. 70s hifi gear was silver and wood. 80s was black. 90s was your choice of silver, black, or champagne. Early 2000s was glossy white. Broadcast/studio AV gear has been gradually getting darker for decades.
Tektronix used turquoise on their cabinets, and not areas where controls and indicators were. I do not care what the cabinet color is, but it matters where there are controls and indicators.
I have included several Tektronix examples below showing high contrast writing with unsaturated color background areas to show control groupings. Some interface compromises had to be made because of limited area.
In other words, your entire premise that test gear (and other electrical products) all used to be light is not even true.
But it is true that human factors engineering was better in the past, and less subject to marketing's style of the year, at least outside of consumer trash.
Me? I like the dark look. But I’m humble enough to recognize that this is simply my personal preference, and don’t try to pretend like there’s some inherent, universal technical advantage to it. (As KungFuJosh says, it can actually be advantageous for some people.)
By and large, I am not a fan of dark mode in software. But even that is situational, not absolute. (I like it for Altium’s user interface — but not for schematics. I like it for YouTube, but hate it for macOS and iOS. ::shrug:: )
I prefer high contrast and maximum visual acuity, which excludes blue and violet around fine details. Dark front panels make my workbench darker, and typically are combined with poor contrast detailing. Fix one or the other, or both.
I would vote for matte aluminium front panels that can be cleaned, will not yellow, good ergonomic contrast.
Tektronix did a lot of that in the past, as shown below. I think they had a screen printing process which was combined with aluminum anodization to make essentially indestructible front panels.
The markings on their buttons and skirts is another matter, but they still held up well enough.