JSFiddle - React, Tailwind, and code Playground

HTML

<a class="read-more" href="#" id="part-a">change buttons</a>
 <div class="content_container">
                        <h2>Food</h2>
                        <p>New York City's food culture includes a variety of international cuisines influenced by the
                            city's immigrant history. Central European and Italian immigrants brought bagels, cheesecake,
                            and New York-style pizza into the city, while Chinese and other Asian restaurants, sandwich
                            joints, trattorias, diners, and coffeehouses have become ubiquitous. </p>
                        <div class="button_small">
                            <a class="read-more" href="#">Read more</a>
                        </div>
                    </div>
                    <div class="content_container">
                        <h2>Museums</h2>
                        <p>New York City is home to hundreds of cultural institutions and historic sites, many of which are
                            internationally known. Museum Mile is the name for a section of Fifth Avenue running from 82nd
                            to 105th streets on the Upper East Side of Manhattan, in an area sometimes called Upper
                            Carnegie Hill.</p>
                        <div class="button_small">
                            <a class="read-more" href="#">Read more</a>
                        </div>
                    </div>

CSS

.button_small {
    font: normal 110% Arial, Helvetica, sans-serif;
    height: 15px;
    width: 80px;
    padding: 5px 2px 7px 2px;
    background: #1D1D1D;
    background: -moz-linear-gradient(#676767, #1D1D1D);
    background: -o-linear-gradient(#676767, #1D1D1D);
    background: -webkit-linear-gradient(#676767, #1D1D1D);
    border-radius: 15px 15px 15px 15px;
    -moz-border-radius: 15px 15px 15px 15px;
    -webkit-border: 15px 15px 15px 15px;
    -webkit-box-shadow: rgba(0, 0, 0, 0.5) 0px 0px 5px;
    -moz-box-shadow: rgba(0, 0, 0, 0.5) 0px 0px 5px;
    box-shadow: rgba(0, 0, 0, 0.5) 0px 0px 5px;
}

JavaScript

document.getElementById('part-a').addEventListener("click", changeButtonStyles, false);

function changeButtonStyles() {
    var actualButtons = document.getElementsByClassName("read-more");
    for (var i = 0; i < actualButtons.length; i++) {
        actualButtons[i].parentNode.className = "basic";
        actualButtons[i].className = "btn btn-xs btn-default";
    }
}