Found a minor bug on desktop startup if you use the xfce session resume:

The icons in the panel module "systray" does not have the theme properly applied when xfsettingsd is starting. I am not sure if this is the case when systray is starting before xfsettingsd or vice versa. XFCE desktop compositor is disabled.

This applies on icons like pidgin or clementine using the systray and are started up by the xfce4-session resume. This bug happens although the GTK-Theme supports the correct coloring for all GTK2/GTK3 items (e.g. Greybird). If the corresponding applications are closed and opened again manually the theme is correctly applied by xfsettingsd. It is also correctly applied if "xfsettingsd --replace" is executed manually. Looks like a tricky timing issue.

Though it might be worth fixing because there are bug reports on themes out there because users think the themes are incomplete because they seem to be not applied on the systray.