Issue #12763: Recognize macOS 'Differentiate without Color' setting
This patch attempts to resolve issue #12763.
macOS has a 'Differentiate without Color' setting, which changes certain options to be more distinct beyond color changes. In GIMP, the main area seems to be the GtkSwitches, which rely on color to show if they're on or off. We have a CSS style that hides I/O images to better match 2.10 style.
This patch moves that style from common.css to /app/gu/themes, and conditions it on the macOS setting. For other OS, the style is added by default.