can't resize Nautilus window in gnome-flashback using Ambiance theme

When I use the Ambiance theme on my Ubuntu 15.10 (wily) in gnome-flashback I can resize the nautilus window only on the upper side (the area to resize the window is very thin). If I use an other theme like Adwaita resizing is possible on all four sides! I only have this problem with nautilus, all other Applications are fine!

Maybe an border is to thin to hit it on an HD Screen (1920x1800px)?!

In Adwaita the Cursor switches to "resize" on all four sides!

How can I resize the Nautilus window in the Ambiance Theme?

Ambiance is designed for Unity and does not work good with CSD windows. Best option probably might be to use theme that has good support for CSD windows.

But if you still want to use Ambiance theme you can create ~/.config/gtk-3.0/gtk.css file with following content:

.window-frame {
    margin: 10px;
}

10px will be available for resizing - 10px are used in Adwaita theme, but you can use smaller or larger size if you want.

Actually, it can be fixed by adding the following to /usr/share/themes/Ambiance/gtk-3.20/gtk-widgets.css for GTK 3.20+ (tested on Ubuntu 16.10):

/* BELOW FIXES RESIZE ISSUE ON SOME WINDOWS */
/* workaround to avoid unwanted black frames if switching compositor on/off */
.background decoration  {
    box-shadow: none;
}

.background.csd decoration {
    border-radius: 6px 6px 0px 0px;
    border-width: 0px;
    box-shadow: 0 0 0 1px @wm_border, 0 2px 8px 3px @wm_shadow;
    /* this is used for the resize cursor area */
    margin: 10px;
}
/* END RESIZE FIX */

If you want to adjust the shadow colors, simply create a color rule for @wm_border and @wm_shadow in gtk-main.css like:

@define-color wm_border @dark_bg_color;
@define-color wm_shadow shade (@wm_border, 1.20)