I used to be possible to use -moz-appearance: -moz-win-glass and transparency fore various elements to create a FF theme that blended in well with the OS theme.
But it’s broken now. -moz-win-glass only results in a static color. Manually, or more likely via the slideshow background feature of Windows.
Now it just (apparently) paints whatever the color was at strtup. So Firefox (including the taskbar) might end up being blue, while every other program’s window is red.
So what’s needed is a CSS color variable that uses the Windows theme’s accent color.