Modifies style values on the widget.
Modifications made using this technique take precedence over style values set via an RC file, however, they will be overridden if a style is explicitly set on the widget using Widget.setStyle. The [GtkRcStyle-struct|GtkRcStyle-struct] is designed so each field can either be set or unset, so it is possible, using this function, to modify some style values and leave the others unchanged.
Note that modifications made with this function are not cumulative with previous calls to Widget.modifyStyle or with such functions as Widget.modifyFg. If you wish to retain previous values, you must first call Widget.getModifierStyle, make your modifications to the returned style, then call Widget.modifyStyle with that style. On the other hand, if you first call Widget.modifyStyle, subsequent calls to such functions Widget.modifyFg will have a cumulative effect with the initial modifications.