На странице стиль минимальный, светлый, premium/editorial. Вставь этот CSS в SimplyBook Custom CSS:

```css
#events {
  font-family: inherit;
  max-width: 920px;
  margin: 40px auto;
  color: #1f1f1f;
}

#events h3,
#eventForm #start_date-block-container h3,
#eventForm #timeline-container h3 {
  font-size: 24px;
  font-weight: 400;
  letter-spacing: .04em;
  text-transform: uppercase;
  color: #111;
  margin-bottom: 18px;
}

#events p.duration,
#events div.duration {
  font-size: 13px;
  letter-spacing: .08em;
  text-transform: uppercase;
  color: #9a7a52;
  margin: 8px 0;
}

#events p.description,
#events div.description {
  font-size: 15px;
  line-height: 1.7;
  color: #555;
}

#events .selectedEvent {
  background: #fffaf3 !important;
  border: 1px solid #d9b98f !important;
  border-radius: 22px;
  box-shadow: 0 18px 45px rgba(0,0,0,.08);
  padding: 28px;
}

#events input.reserve_time_btn,
#eventForm #save_button {
  background: #111 !important;
  color: #fff !important;
  border: 1px solid #111 !important;
  border-radius: 999px !important;
  padding: 14px 34px !important;
  font-size: 13px !important;
  letter-spacing: .12em;
  text-transform: uppercase;
  cursor: pointer;
  transition: all .25s ease;
}

#events input.reserve_time_btn:hover,
#eventForm #save_button:hover {
  background: #9a7a52 !important;
  border-color: #9a7a52 !important;
}

#events input.select_another_btn {
  background: transparent !important;
  color: #111 !important;
  border: 1px solid #d8d0c5 !important;
  border-radius: 999px !important;
  padding: 12px 28px !important;
  font-size: 12px !important;
  letter-spacing: .1em;
  text-transform: uppercase;
}

div.ui-widget-content {
  background: #fff !important;
  border: 1px solid #eee2d4 !important;
  border-radius: 20px !important;
  box-shadow: 0 14px 38px rgba(0,0,0,.06);
}

div.ui-widget-header {
  background: #f8f2ea !important;
  border: none !important;
  color: #111 !important;
  font-weight: 400 !important;
  letter-spacing: .08em;
  text-transform: uppercase;
}

#timeline-container table.timeline {
  border-collapse: separate !important;
  border-spacing: 8px !important;
  width: 100%;
}

.timeline td.not_worked_time {
  background: #f3f3f3 !important;
  color: #bbb !important;
  border-radius: 12px;
}

.timeline td.free_time {
  background: #fffaf3 !important;
  border: 1px solid #e4caa8 !important;
  color: #111 !important;
  border-radius: 12px;
  cursor: pointer;
  transition: all .2s ease;
}

.timeline td.free_time:hover {
  background: #ead4b8 !important;
}

.timeline td.selected_time {
  background: #111 !important;
  color: #fff !important;
  border-radius: 12px;
}

.timeline td.reserved_time {
  background: #eee !important;
  color: #aaa !important;
  border-radius: 12px;
  text-decoration: line-through;
}

div#loading {
  color: #9a7a52 !important;
  font-size: 14px;
  letter-spacing: .08em;
  text-transform: uppercase;
}

#start_date-block-container .zend_form dt,
#start_date-block-container .zend_form dt b,
#start_date-block-container .zend_form dd label {
  font-size: 14px !important;
  color: #333 !important;
  letter-spacing: .03em;
}
```

Я бы начал именно с этой версии: она подстраивается под светлый премиальный стиль страницы Packages и не выглядит как “встроенный чужой виджет”.