Currently _GTK_THEME_VARIANT property is set only if theme variant is dark, otherwise property is removed. Please consider setting this property always even when theme variant is light/default.

The problem is that currently there is no way to detect if window does not support theme variants or light theme variant is requested.

There is at least one case where this can be considered as bug:
1) User has enabled Global Dark Theme
2) User has set GNOME Terminal to use light theme variant
3) Mutter shows dark theme decoration, but window itself use correct - light theme variant.

It would be nice if it could work like this:
1) If windows does not have _GTK_THEME_VARIANT - window manager can use default theme variant - light or dark if it is enabled globally (in settings.ini)
2) If property is set - then use variant based on value it has. empty string should be considered as light/normal theme variant.

diff for gdkwindow-x11.c:
-  gdk_x11_window_set_utf8_property (window, "_GTK_THEME_VARIANT", variant);
+  gdk_x11_window_set_utf8_property (window, "_GTK_THEME_VARIANT",
+                                    variant ? variant : "");