Nobody said that we need a flat look. But if any of the controls fail to meet sufficient contrast in dark mode that is a bug. Not a design decision.
Not that the color scheme you had is a good choice for good contrast, but still.
The problem with your Haiku theme is really low contrast. Since panels are darker in MacOS theme contrast is better. Fonts are even too bright in some places.
add to this that above Github.com uses their own proper dark theme. But falcon does not support this, so there is some addon that replaces the colors in wierd ways.
WebPositive does support the proper dark mode for webpages.
Yes, we can’t get a flat look from the default control look. That’s why there is a different control look for that now.
But still, that has nothing to do with a dark theme. We could change how the gradients are generated, to make them less steep. In fact there is already a “shine” color in the color preferences, that could be used for this, and so it could be adjusted from the color preferences. But the current look does not use it.