gtk: Add a forgotten backdrop titlebar change

src/_sass/gtk/apps/_unity.scss

@@ -31,7 +31,10 @@ UnityDecoration {

                                
                                
                                
                            
                                
                                    
                                        
                                            31  
                                                background-color: $titlebar;
                                        
                                        
                                            
                                            
                                            
                                            
                                        
                                    
                                
                                
                                
                            
                                
                                    
                                        
                                            32  
                                                color: $titlebar-text;
                                        
                                        
                                            
                                            
                                            
                                            
                                        
                                    
                                
                                
                                
                            
                                
                                    
                                        
                                            33  
                                            
                                        
                                        
                                            
                                            
                                            
                                            
                                        
                                    
                                
                                
                                
                            
                                
                                    
                                        34 -
                                            &:backdrop { color: $titlebar-text-secondary; }
                                        
                                        
                                        
                                    
                                
                                
                                
                            
                                
                                    
                                        34 +
                                            &:backdrop {
                                        
                                        
                                        
                                    
                                
                                
                                
                            
                                
                                    
                                        35 +
                                              background-color: $titlebar-backdrop;
                                        
                                        
                                        
                                    
                                
                                
                                
                            
                                
                                    
                                        36 +
                                              color: $titlebar-text-secondary;
                                        
                                        
                                        
                                    
                                
                                
                                
                            
                                
                                    
                                        37 +
                                            }
                                        
                                        
                                        
                                    
                                
                                
                                
                            
                                
                                    
                                        
                                            38  
                                              }
                                        
                                        
                                            
                                            
                                            
                                            
                                        
                                    
                                
                                
                                
                            
                                
                                    
                                        
                                            39  
                                            
                                        
                                        
                                            
                                            
                                            
                                            
                                        
                                    
                                
                                
                                
                            
                                
                                    
                                        
                                            40  
                                              .left,