matrixion/resources/styles/system.qss

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

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

#sideBar {
    border: none;
    border-right: 1px solid palette(text);
    border-left: 1px solid palette(text);
}

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

QuickSwitcher {
    background-color: palette(window);
}

DateSeparator {
    qproperty-textColor: palette(text);
    qproperty-boxColor: palette(window);
}

SuggestionsPopup {
    background-color: palette(window);
}

PopupItem {
    background-color: palette(window);
    qproperty-hoverColor: rgba(192, 193, 195, 120);
}

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

FileItem {
    qproperty-textColor: palette(text);
    qproperty-backgroundColor: palette(base);
    qproperty-iconColor: palette(window);
}

AudioItem {
    qproperty-textColor: palette(text);
    qproperty-backgroundColor: palette(base);
    qproperty-iconColor: palette(window);
}

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(light);
    qproperty-backgroundColor: palette(window);

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

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

    qproperty-btnColor: palette(mid);
    qproperty-btnTextColor: palette(text);

    qproperty-timestampColor: palette(text);
    qproperty-highlightedTimestampColor: palette(light);

    qproperty-avatarBgColor: palette(mid);
    qproperty-avatarFgColor: palette(text);

    qproperty-bubbleFgColor: palette(text);
    qproperty-bubbleBgColor: palette(mid);
}

CommunitiesListItem {
    qproperty-highlightedBackgroundColor: palette(highlight);
    qproperty-hoverBackgroundColor: palette(light);
    qproperty-backgroundColor: palette(window);

    qproperty-avatarBgColor: palette(mid);
    qproperty-avatarFgColor: palette(text);
}

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

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

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

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

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

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

ScrollBar {
    qproperty-handleColor: palette(text);
    qproperty-backgroundColor: palette(window);
}

FloatingButton {
    qproperty-backgroundColor: palette(window);
    qproperty-foregroundColor: palette(text);
}

QListWidget {
    background-color: palette(window);
    color: palette(text);
}