The line is almost at the end of the core-standard.qss file. The HEX in red below is the one that needs changing;
Interface Window
QWidget#IWContainer, QWidget#ICContainer, QWidget#RTPContainer {
background:
#A0A0A0;
color: white;
font-family: "<core_sans_font>", DejaVu Sans, Vera Sans, Verdana, Helvetica, Sans Serif;
font-size: 8pt;