Slight adjustment to custom styling example: source view slider is now shown

docs/custom_styling.md

@@ -38,11 +38,11 @@ was made with the following ~/.config/gtk-3.0/gtk.css:
   background: transparent;
 }
 
-.juci_notebook {
+.juci_notebook :not(slider) {
   background-color: rgba(255, 255, 255, 0.8);
 }
 
-.juci_notebook * {
+.juci_notebook * :not(slider) {
   background: transparent;
 }