GtkNotebook::set_tab_hborder
void set_tab_hborder(int tab_hborder); 
    
     Sets the left and right borders to be tab_hborder 
     pixels wide for all the notebook's tabs.
    
    
     Combined with set_tab_vborder() 
     this is the suggested method for setting tab borders.
    
    
     See also: set_tab_border() ,
     set_tab_vborder()