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CSS

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HTML 

              
                    <div id="container" class="container">
    <div id="tabs" class="tabs-container">

        <div class="tabs">
            <a id="tab1" data-tab="1" class="tab">Mercury</a>
            <a id="tab2" data-tab="2" class="tab">Venus</a>
            <a id="tab3" data-tab="3" class="tab">Earth</a>
            <a id="tab4" data-tab="4" class="tab">Mars</a>
        </div>

        <div class="content">
            <div id="tabcontent1" data-tab="1" class="tabcontent">
              <p>Mercury is the smallest and innermost planet in the Solar System.</p>
              <p>Its orbit around the Sun takes 87.97 days, the shortest of all the planets in the Solar System.</p>
              <p>It is named after the Roman deity Mercury, the messenger of the gods.</p>
              <p class="read-more-link"><a href="https://en.wikipedia.org/wiki/Mercury_(planet)" target="_blank">Learn more<span class="icon">➡️</span></a></p>
            </div>
            <div id="tabcontent2" data-tab="2" class="tabcontent">
                  <p>Venus is the second planet from the Sun. It is named after the Roman goddess of love and beauty.</p>
                  <p>Venus orbits the Sun every 224.7 Earth days.</p>
                 <p>As the second-brightest natural object in the night sky after the Moon, Venus can cast shadows and, rarely, is visible to the naked eye in broad daylight.</p>
                  <p class="read-more-link"><a href="https://en.wikipedia.org/wiki/Venus" target="_blank">Learn more<span class="icon">➡️</span></a></p>
            </div>
            <div id="tabcontent3" data-tab="3" class="tabcontent">
                <p>Earth is the third planet from the Sun.</p>
                <p>Earth orbits around the Sun in 365.256 days, a period known as an Earth sidereal year. During this time, Earth rotates about its axis about 366.256 times.</p>
              <p>Earth's axis of rotation is tilted with respect to its orbital plane, producing seasons on Earth.</p>
                  <p class="read-more-link"><a href="https://en.wikipedia.org/wiki/Earth" target="_blank">Learn more<span class="icon">➡️</span></a></p>
            </div>
            <div id="tabcontent4" data-tab="4" class="tabcontent">
                <p>Mars is the fourth planet from the Sun and the second-smallest planet in the Solar System after Mercury.</p>
                <p> In English, Mars carries a name of the Roman god of war and is often referred to as the 'Red Planet'. The latter refers to the effect of the iron oxide prevalent on Mars' surface.</p>
                <p>Mars is a terrestrial planet with a thin atmosphere.</p>
                  <p class="read-more-link"><a href="https://en.wikipedia.org/wiki/Mars" target="_blank">Learn more<span class="icon">➡️</span></a></p>
            </div>
        </div>

    </div>
  
    <div class="dark-mode-switch">
        <label class="switch-label" for="dark-mode-switch">Dark mode</label>
        <label class="switch">
            <input type="checkbox" id="dark-mode-switch">
            <span class="slider round"></span>
        </label>
    </div>
  
</div>





<!-- ---------------------- -->

<div class="small-screen">
    <div class="small-screen-content tabcontent">
        <p>You're going to need a bigger display my friend.</p>
        <p class="read-more-link"><a href="https://www.samsung.com/uk/monitors/monitor-c49j89-lc49j890dkuxen/" target="_blank">Learn more<span class="icon">➡️</span></a></p>
    </div>
</div>
!

CSS 

              
                /* Import Google Font */
@import url(https://fonts.googleapis.com/css?family=Nunito+Sans);

/* RESET */
* {
    box-sizing: border-box;
    margin: 0;
    padding: 0;
}

/* STYLING */
.container {
    display: flex;
    height: 100vh;
    background-color: #f4f4f4;
    color: #a2a2a2;
    font-family: "Nunito Sans", Arial, Helvetica, sans-serif;
}
.darkmode .container {
    background-color: #1A1B1F;
    color: #8a8a8a;
}
.tabs-container { 
    width: 540px;
    max-width: 620px; 
    min-width: 420px;
    margin: auto;
}

/* Style the tabs */
.tabs {
    margin-bottom: 28px; 
    display: flex;
      justify-content: space-between;
}

.tabs a {
    cursor: pointer;
    padding: 12px 24px;
    width: 120px;
    text-align: center;
    font-weight: bold;
    border-radius: 18px;
    transition: background 0.1s, color 0.1s;
    background: linear-gradient(145deg, #ffffff, #dcdcdc);
    box-shadow:  3px 3px 5px #bebebe, 
                 -3px -3px 5px #ffffff;
}
.darkmode .tabs a {
    background: linear-gradient(145deg, #1c1d21, #17181c);
    box-shadow:  3px 3px 6px #101114, 
                 -3px -3px 6px #24252a;
}

/* Change background color of tabs on hover */
.tabs a:hover {
    background: linear-gradient(145deg, #f4f4f4, #cecece);
    color: #888;
}
.darkmode .tabs a:hover {
    background: #141414;
    color: #bbb;
}

/* Styling for active tab */
.tabs a.active {
    background-color: #f4f4f4;
    color: #bdbdbd;
    cursor: default;
    padding: 14px 22px 10px 26px;
    background: #f4f4f4;
    box-shadow: inset 3px 3px 5px #cbcbcb, 
                inset -3px -3px 5px #ffffff;
}
.darkmode .tabs a.active {
    background: #1A1B1F;
    box-shadow: inset  3px 3px 6px #101114, 
                inset -3px -3px 6px #24252a; 
    color: #6a6a6a;
}


/* Style the tab content */
.tabcontent {
    padding: 46px;
    min-height: 288px;
    display: none;    
    border-radius: 18px;
    background: #f4f4f4;
    box-shadow:  3px 3px 6px #bebebe, 
                 -3px -3px 6px #ffffff;
}
.darkmode .tabcontent {
    background: linear-gradient(145deg, #1c1d21, #17181c);
    box-shadow:  3px 3px 6px #101114, 
                 -3px -3px 6px #24252a;
}
.content .active {
    display: block;
}
.tabcontent p {
    margin-bottom: 12px;
}
.tabcontent p:last-child {
    margin-bottom: 0;
}
.tabcontent .read-more-link a {
    color: #626262;
    text-decoration: none;
    font-size: 0.85em;
    font-weight: bold;
}
.darkmode .tabcontent .read-more-link a {
    color: #d4d4d4;
}
.icon {
    padding-left: 8px;
    font-size: 16px;
}

/* THE DARKMODE SWITCH */ 
.dark-mode-switch {
    position: absolute;
    top: 16px;
    right: 16px;
}
.dark-mode-switch .switch {
/*     margin-left: 4px; */
}
.switch-label {
    cursor: pointer;
    font-size: 0.85em;
}
/* the box around the slider */
.switch {
    position: relative;
    display: inline-block;
    width: 44px;
    height: 22px;
    margin-left: 4px;
}
/* Hide default HTML checkbox */
.switch input {
    opacity: 0;
    width: 0;
    height: 0;
}
/* The slider */
.slider {
    position: absolute;
    cursor: pointer;
    top: 0;
    left: 0;
    right: 0;
    bottom: 0;
    background-color: #1A1B1F;
    transition: .2s;
    box-shadow:  2px 2px 3px #ffffff, 
                 -2px -2px 3px #bebebe;
}
.darkmode .slider {
    box-shadow:  2px 2px 3px #34353a, 
                 -2px -2px 3px #000104;
}
.slider:before {
    position: absolute;
    content: "";
    height: 18px;
    width: 18px;
    left: 3px;
    bottom: 2px;
    background: #9294b8;
    -webkit-transition: .4s;
    transition: .4s;
}
input:checked + .slider {
    background-color: #f4f4f4;
}
input:checked + .slider:before {
    transform: translateX(21px);
}
/* Rounded sliders */
.slider.round {
    border-radius: 11px;
}
.slider.round:before {
    border-radius: 50%;
}



/*==============================*/
.small-screen {
    display: none;
    background-color: #f4f4f4;
    height: 100vh;
    color: #a2a2a2;
    font-family: "Nunito Sans", Arial, Helvetica, sans-serif;
}
.darkmode .small-screen {
    background-color: #1A1B1F; 
    color: #8a8a8a;
}
.small-screen-content {
    width: 70%;
    margin: auto;
}
@media only screen and (max-width: 600px) {
    .container {
        display: none;
    }
    .small-screen {
        display: flex;
    }
    .tabcontent {
        display: block;
    }
}
!

JS 

              
                // DOM Elements
const tabs = document.querySelectorAll('.tab')
const tabContents = document.querySelectorAll('.tabcontent')
const darkModeSwitch = document.querySelector('#dark-mode-switch')
  
// Functions
const activateTab = tabnum => {
    
    tabs.forEach( tab => {
      tab.classList.remove('active')
    })
    
    tabContents.forEach( tabContent => {
        tabContent.classList.remove('active')
    })
  
    document.querySelector('#tab' + tabnum).classList.add('active')
    document.querySelector('#tabcontent' + tabnum).classList.add('active')
    localStorage.setItem('jstabs-opentab', JSON.stringify(tabnum))
  
}

// Event Listeners
tabs.forEach(tab => {
    tab.addEventListener('click', () => {
        activateTab(tab.dataset.tab)
    })
})

darkModeSwitch.addEventListener('change', () => {
    document.querySelector('body').classList.toggle('darkmode')
    localStorage.setItem('jstabs-darkmode', JSON.stringify(!darkmode))
})

// Retrieve stored data
let darkmode = JSON.parse(localStorage.getItem('jstabs-darkmode'))
const opentab =  JSON.parse(localStorage.getItem('jstabs-opentab')) || '3'

// and..... Action!
if (darkmode === null) {
    darkmode = window.matchMedia("(prefers-color-scheme: dark)").matches // match to OS theme
}
if (darkmode) {
    document.querySelector('body').classList.add('darkmode')
    document.querySelector('#dark-mode-switch').checked = 'checked'
}
activateTab(opentab)
!
999px

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