/usr/share/themes/Gray/gtk-2.0/panel.rc is in gray-theme 0.1-0ubuntu2.
This file is owned by root:root, with mode 0o644.
The actual contents of the file can be viewed below.
| 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 61 62 63 64 65 66 67 68 69 70 71 72 73 74 75 76 77 78 79 80 81 82 83 84 85 86 87 88 89 90 91 92 93 94 95 96 97 98 99 100 101 102 103 104 105 106 107 108 109 110 | #################### PANEL BACKGROUND #########################xx
style "panelbg"
{
  xthickness            			= 0
  ythickness            			= 0
 bg_pixmap[NORMAL]					= "Panel/panel-bg.png"
#bg_pixmap[SELECTED]					= "Panel/panel-bg.png"
#bg_pixmap[INSENSITIVE]					= "Panel/panel-bg.png"
#bg_pixmap[PRELIGHT]					= "Panel/panel-bg.png"
 
}
class "*Panel*" style "panelbg"
class "*notif*" style "panelbg"
class "*Notif*" style "panelbg"
class "*Tray*" style "panelbg"
class "*tray*" style "panelbg"
##################### PANEL BUTTONS ###############################
style "panelbuttons" = "default"
{
  fg[NORMAL]        = "#ffffff" # very dark brown
  fg[PRELIGHT]      = "#ffffff" # text on buttons (hover)
  fg[ACTIVE]        = "#ffffff" # text on unfocused tabs
  fg[SELECTED]      = "#ffffff" # selected text on lists
  fg[INSENSITIVE]   = "#ffffff" # greyed "unused" text
 
 xthickness            			= 2
  ythickness            			= 1
	GtkWidget::focus_padding = 2
	engine "pixmap" {
      
		image
		{
			function        			= BOX
			recolorable     		= TRUE
			state										= NORMAL
			file            						= "Panel/panelbutton1.png"
			border          			= { 4, 4, 4, 4 }
			stretch         				= TRUE
		}
		
image
		{
			function        			= BOX
			recolorable     		= TRUE
			state										= PRELIGHT
			file            						= "Panel/panelbutton2.png"
			border          			= { 4, 4, 4, 4 }
			stretch         				= TRUE
		}
		image
		{
			function        			= BOX
			recolorable     		= TRUE
			shadow							= OUT
			state										= PRELIGHT
			file            						= "Panel/panelbutton1.png"
			border          			= { 4, 4, 4, 4 }
			stretch         				= TRUE
			#overlay_file   			= "panelbutton2.png"
			#overlay_border	= { 4, 4, 4, 4 }
			#overlay_stretch	= TRUE
		}
		
		image
		{
			function        			= BOX
			recolorable     		= TRUE
			shadow							= IN
			state										= PRELIGHT
			file            						= "Panel/panelbutton3.png"
			border          			= { 4, 4, 4, 4 }
			stretch         				= TRUE
			#overlay_file     		= "panelbutton2.png"
			#overlay_border	= { 4, 4, 4, 4 }
			#overlay_stretch	= TRUE
		}
		
		image
		{
			function        			= BOX
			recolorable     		= TRUE
			state										= ACTIVE
			file            						= "Panel/panelbutton3.png"
			border          			= { 4, 4, 4, 4 }
			stretch         				= TRUE
		}  
image
		{
			function        			= BOX
			recolorable     		= TRUE
			state										= INSENSITIVE
			file            						= "Panel/panelbutton2.png"
			border          			= { 4, 4, 4, 4 }
			stretch         				= TRUE
		}  
	}
}
widget_class "*Panel*GtkToggleButton" style "panelbuttons"
widget_class "*Panel*GtkButton" style "panelbuttons"
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