Custom colors worked in the vast majority of the post-95 Windows apps I ever used. Some of them did custom theming that ignored system colors, but a lot of thos

Custom colors worked in the vast majority of the post-95 Windows apps I ever used. Some of them did custom theming that ignored system colors, but a lot of those had swappable themes so you could just get a custom theme that was dark. GTK and QT all typically obeyed Windows system colors when I used apps built with those. I don't know for certain that the JVM's UI toolkits obeyed them, but I'm pretty sure they did. The Windows "high contrast" accessibility mode relied on system colors so you would expect them to work. The modern hell era of 'every UI element is a png file' is specifically the fault of Apple who caused it (with iOS's gaudy visual style that could not be efficiently rasterized using vector graphics by the average developer) and reinforced it with the way they handled the introduction of Retina displays (adding specific tooling to support tons of massive resolution-specific pngs that developers then shipped in their apps) System colors were very easy to tap into using VB6, Delphi, VB.Net, C#, etc. If people weren't obeying system colors they just didn't care about obeying user intent or about helping people with visual impairment.