style "compact-toolbar" {
GtkToolbar::internal-padding = 0
xthickness = 1
ythickness = 1
}
style "compact-button" {
xthickness = 0
ythickness = 0
}
class "GtkToolbar" style "compact-toolbar"
widget_class "*<GtkToolbar>*<GtkButton>" style "compact-button"
place this in the ～/.gtkrc-2.0 file
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Thanks to Azuma59.