matrixion/resources/styles/system.qss

OverlayWidget,
OverlayWidget > * {
    background-color: palette(window);
}

#mainContent,
#mainContent > * {
    background-color: palette(base);
}

TimelineView,
TimelineView > *,
TimelineItem,
TimelineItem > * {
    background-color: palette(base);
}

FlatButton {
    qproperty-foregroundColor: palette(text);
}

RoomInfoListItem {
    qproperty-highlightedBackgroundColor: palette(highlight);
    qproperty-hoverBackgroundColor: palette(dark);
    qproperty-backgroundColor: palette(window);

    qproperty-titleColor: palette(text);
    qproperty-subtitleColor: palette(text);

    qproperty-highlightedTitleColor: palette(text);
    qproperty-highlightedSubtitleColor: palette(text);
}

LoadingIndicator {
    qproperty-color: palette(text);
}

#ChatPageLoadSpinner {
    qproperty-color: #acc7dc;
}

UserInfoWidget,
UserInfoWidget > * {
    background-color: palette(window);
}