matrixion/resources/styles/system.qss

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

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

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

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

RaisedButton {
    qproperty-foregroundColor: palette(light);
}

TextField {
    qproperty-backgroundColor: palette(window);
}

WelcomePage,
LoginPage,
RegisterPage {
    background-color: palette(window);
}

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

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

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

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

#ChatPageLoadSpinner {
    qproperty-color: palette(light);
}

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

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

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

QTextEdit,
QLineEdit {
    background-color: palette(window);
}