smartschool-platform/frontend/pc-web/app/view/Interaction/timetable/Detail.scss

.timetable {
    .x-grid-cell.x-period-column {
        &>div {
            padding: 0;
            &>div {
                &>div>div {
                    a.x-btn,a.x-btn-over,a.x-btn-pressed {
                        background: #ffffff;
                        span {
                            color: #404040 !important;
                        }
                    }
                }
            }
        }
    }
}