Xfce4-Panel Adds Dark Mode Preference

As a change for next year's Xfce 4.16, the xfce4-panel now exposes a preferences option for those preferring "dark modes" of GTK themes.

Landing this week in xfce4-panel was this commit providing a dark mode preference for Xfce4, similar to the growing trend with other desktops/UIs for those wanting a "dark mode" interface.

Enabling this option will request the GTK dark theme variant of capable themes. For a long time now GTK has exposed a property (gtk-application-prefer-dark-theme) for preferring dark themes while now is being tapped by xfce4-panel.

This change comes not long after the recent Xfce 4.14 release while Xfce 4.16 is expected in 2020 as an evolutionary update to this lightweight GTK desktop that will focus on code cleaning and laying the groundwork for other improvements in post-4.16 releases.