And vice versa for us dark users.
Well, most pages are still light by default nowadays, and I don't think there is a big momentum to change the default. But that's not my point.
I remember back when websites had more control of your scrollbar size, color and other such browser functions and sometimes it was a useability and accessibility nightmare so I'd argue that the browser, which you can control for consistency and accessibility, should have reigning control over the scrollbar and not the content.
Content still has such control. They used to be hacky, hiding the scrollbar via
overflow: hidden etc and create their own scrollbar, which causing lots of usability and accessibilty issues.
It's not that bad nowadays. Many websites now use
::-webkit-scrollbar pseudo-elements for that on WebKit/Blink-based browsers, which is not ideal but not too bad. There is also a new mechanism called CSS Scrollbars, which currently only Firefox supports, allowing content to customize the scrollbar color and thickness to some extent.
Websites can use these tools to make their scrollbars fit their theme better, without sacrificing lots of usability and accessibility (unless they really want to, which they can always do anyway).
Context menus and popups will apparently be darkened in the future according to bug 1553682 and if you consider those part of the browser and that they should be thematically consistent with the browser, I'd also argue that scrollbars are in the same category of a useability feature that should be controlled by the user and browser, not by third parties and accordingly thematically consistent with the browser.
One of the difference is probably that they don't show all the time, while scrollbars do. Also websites can customize their context menu and popups as well, and more easily, and many of them have already been doing so.
Perhaps overlay scrollbars could also be implemented, as is a beta feature in Chrome and Vivaldi, since they're aesthetically very neutral and far less distracting from content no matter the theme as well not hindering accessibility either, from my useability perspective.
Overlay scrollbars are not the one-setting-fit-all solution either. If you had a look at macOS's overlay scrollbar, it uses different color pairs for light and dark background. So there still need to be different settings for different content.
That reminds me, though, that our Cocoa widget does support light and dark scrollbars, and automatically picks one based on the background color of the element it's showing for (unless overridden by the content). We can probably do something similar on other widgets on top of the custom scrollbar infrastructure.
It's not perfect (e.g. if there is a background image way different than the background color, or the element is almost completely covered by its descendant which has a very different color settings), but should be good enough for majority of cases.