JSFiddle - React, Tailwind, and code Playground

HTML

<div class="menu">
  <span class="menu-item checked">Главная</span>
  <span class="menu-item">Продукция</span>
  <span class="menu-item">Доставка</span>
  <span class="menu-item">Контакты</span>
</div>

CSS

.menu {
  display: flex;
  background-color: #01d7b8;
  font-size: 18px;
}

.menu-item {
  position: relative;
  text-transform: uppercase;
  font-family: consolas;
  flex: 1 0 25%;
  text-align: center;
  line-height: 2em;
}

.menu-item.checked {
  color: #fff;
  background-color: #2978ed;
}

.menu-item.checked:not(:last-child):after {
  content: '';
  position: absolute;
  left: 100%;
  height: 0;
  width: 0;
  border-left: 1em solid #2978ed;
  border-right: 1em solid transparent;
  border-bottom: 1em solid transparent;
  border-top: 1em solid transparent;
}