Change tooltip corner radius to 4px.

resources/gtkrc

 gtk_color_scheme = "fg_color:#000\nbg_color:#F7F7F7\nbase_color:#F7F7F7\ntext_color:#000\nselected_bg_color:#000\nselected_fg_color:#000\ntooltip_bg_color:#F7F7F7\ntooltip_fg_color:#000\n"
 
 gtk-font-name="ChromeDroidSans 9"
 
 # Disable button images.
 gtk-button-images = 0
 
 # Diable mnemonics for labels and menu items.
 gtk-enable-mnemonics = 0
 
@@ skipping 91 matching lines @@
   # Padding for tabs.
   GtkNotebook::tab-curvature = 10
 }
 
 style "TextStyle" = "default" {
   base[SELECTED]      = "#DCE4FA"
 }
 
 style "TooltipStyle" = "default" {
   # Padding to allow a rounded rect border.
-  xthickness = 10
+  xthickness = 8
 
   # Padding to give more space at top and bottom.
-  ythickness = 6
+  ythickness = 4
 
   # Yellowish background.
   bg[NORMAL] = "#FFF9E3"
 
   # Give tooltips a slightly smaller font.
   font_name = "ChromeDroidSans 8"
 }
 
 style "WhiteBackgroundStyle" = "default" {
   bg[NORMAL]        = "#FFF"
@@ skipping 13 matching lines @@
 
 class "GtkTextView" style "TextStyle"
 
 class "GtkViewsTextView" style "TextStyle"
 
 widget "gtk-tooltip*" style "TooltipStyle"
 
 widget "*.chromeos-options-tab" style "WhiteBackgroundStyle"
 
 widget "*.chromeos-options-tab*.GtkEventBox" style "WhiteBackgroundStyle"