JSFiddle - React, Tailwind, and code Playground

HTML

<svg height="0" width="0" style="position: absolute; margin-left: -100%;">
  
  <defs>
    
    <filter id='shadow'>
      <feComponentTransfer in="SourceGraphic">
        <feFuncR type="discrete" tableValues="0"/>
        <feFuncG type="discrete" tableValues="0"/>
        <feFuncB type="discrete" tableValues="0"/>
      </feComponentTransfer>
      <feGaussianBlur stdDeviation="1"/>
       <feComponentTransfer><feFuncA type="linear" slope="0.2"/></feComponentTransfer>
      <feOffset dx="5" dy="1" result="shadow"/>
      <feComposite in="SourceGraphic" />
    </filter>
        
    <linearGradient id="tab-1-bg" x1="0%" y1="0%" x2="0%" y2="65%">
      <stop offset="0%" style="stop-color: rgba(136, 195, 229, 1.0);" />
      <stop offset="100%" style="stop-color: rgba(118, 160, 192, 1.0);" />
    </linearGradient>
    
    <linearGradient id="tab-2-bg" x1="0%" y1="0%" x2="0%" y2="65%">
      <stop offset="0%" style="stop-color: rgba(149, 190, 233, 1.0);" />
      <stop offset="100%" style="stop-color: rgba(112, 153, 213, 1.0);" />
    </linearGradient>
    
    <linearGradient id="tab-3-bg" x1="0%" y1="0%" x2="0%" y2="65%">
      <stop offset="0%" style="stop-color: rgba(61, 149, 218, 1.0);" />
      <stop offset="100%" style="stop-color: rgba(43, 130, 197, 1.0);" />
    </linearGradient>
    
    <linearGradient id="tab-4-bg" x1="0%" y1="0%" x2="0%" y2="65%">
      <stop offset="0%" style="stop-color: rgba(72, 204, 243, 1.0);" />
      <stop offset="100%" style="stop-color: rgba(71, 194, 243, 1.0);" />
    </linearGradient>
      
  </defs>
  
  <path id="tab-shape" class="tab-shape" d="M116.486,29.036c-23.582-8-14.821-29-42.018-29h-62.4C5.441,0.036,0,5.376,0,12.003v28.033h122v-11H116.486
			z">
  
</svg>

<nav role="navigation" class="main-navigation">
  <ul>
    <li class="tab-1 active" data-bg-color="rgba(118, 160, 192, 1.0)">
      <a href="#home">
        <span>Home</span>
        <svg viewBox="0 0 122 40">
           <use xlink:href="#tab-shape"></use>
     ...

CSS

@import url(http://fonts.googleapis.com/css?family=Oswald);
@import url(http://fonts.googleapis.com/css?family=Cabin);

* {
  box-sizing: border-box;
}
body {
  padding: 1rem 3rem;
  font-family: 'Cabin', sans-serif;
  line-height: 1.4;
  min-width: 925px;
}
a {
  text-decoration: none;
}

.main-navigation {
  overflow: hidden;
  position: relative;
  padding: 0 0 0 1rem;
  ul {
    list-style: none;
    padding: 0;
  }
  li {
    float: left;
    width: 12rem;
    height: 5rem;
    margin: 0 0 0 -1rem;
    position: relative;
    &.active {
      z-index: 6; 
    }
  }
  a {
    position: relative;
    display: inline-block;
    color: white;
    width: 100%;
    height: 100%;
    font-family: 'Oswald', sans-serif;
    text-transform: uppercase;
    font-size: 1.4rem;
    text-shadow: 0 1px 0 rgba(0, 0, 0, 0.4);
  }
  svg {
    width: 120%;
    height: 100%;
    // for tab shadows
    // also screws up Firefox
    // filter: url(#shadow);
    position: absolute;
    top: 0;
    left: 0;
    z-index: 1;
    pointer-events: none;
  }
  span {
    position: relative;
    padding: 1rem 0 0 3.3rem;
    z-index: 2;
    display: inline-block;
    width: 100%;
    height: 100%;
  }
  .tab-1 {
    z-index: 4;
    svg {
      fill: url(#tab-1-bg); 
    }
  }
  .tab-2 {
    z-index: 3;
    svg {
      fill: url(#tab-2-bg); 
    }
  }
  .tab-3 {
    z-index: 2;
    svg {
      fill: url(#tab-3-bg);
    }
  }
  .tab-4 {
    z-index: 1;
    svg {
      fill: url(#tab-4-bg);
    }
  }
/*  &::after {
    content: "";
    position: absolute;
    bottom: 16px;
    left: 0;
    width: 798px;
    height: 12px;
    background: linear-gradient(rgba(0, 0, 0, 0), rgba(0, 0, 0, 0.2));
    z-index: 5;
  }*/
}

.main-content {
  position: relative;
  z-index: 10;
  background: rgba(118, 160, 192, 1.0);
  margin: -1.6rem 0 0 0;
  padding: 3rem;
  border-radius: 15px;
  border-top-left-radius: 0;
}

.article {
  background: white;
  padding: 5rem;
  border-radius: 8px;
  h2 {
   ...

JavaScript

var menuItems = $('.main-navigation li');

menuItems.on("click", function(event) {
    
  menuItems.removeClass("active");
  
  $(this).addClass("active");
  
  $(".main-content").css({
    "background": $(this).data("bg-color")
  });
  
  event.preventDefault();
});