Stylish-for-Riot.css

.mx_MatrixChat>:not(.mx_LeftPanel_container):not(.mx_ResizeHandle) {
    background: linear-gradient(rgba(0, 0, 0, 0.45), rgba(24, 27, 33, 1)), url("https://i.imgur.com/lcuHKku.png");
    background-size: cover;
}

.mx_EventTile.focus-visible:focus-within .mx_EventTile_line, .mx_EventTile.mx_EventTile_actionBarFocused .mx_EventTile_line, .mx_EventTile:hover .mx_EventTile_line {
    background-color: #22262e;
    background-color: rgba( 0, 0, 0, 0.30 );
}

.mx_EventTile_highlight .markdown-body .mx_EventTile_line, .mx_EventTile_highlight .mx_EventTile_line{
    background-color: #22262e;
    background-color: rgba( 30, 0, 0, 0.30 );
}

.mx_RoomHeader.light-panel {
    background-color: #22262e;
    background-color: rgba(24, 27, 33, 0.45);
}

.mx_RoomHeader_buttons {
    background-color: #181b21;
    background-color: rgba( 0, 0, 0, 0 );
}

.mx_SenderProfile>span {
    background-color: #22262e;
    padding: 0 10px;
}

.mx_RoomView .mx_MessageComposer {
    padding-bottom: 2rem;
}