I also have tried modifying light.css file (theme css file) with the following:
QToolBar > QToolButton:checked {
border-image: url(':/icons/light/overview') 0 0 0 0 stretch stretch;
background-color:opacity:0.52;
border:20px solid #282828;
color:#fff;
font-weight:bold;
}
It did not change anything at all too.