无法居中容器

Question

我正在尝试使容器居中，并且正在使用 twitter bootstrap 4

这是html

<div class="container">
            <div class="features">
                <h3 class="text-center mb-5">So many ways to connect with all your friends</h1>
            <div class="row">
        <div class="col-sm-4">
            <div class="card" style="width: 18rem;">
  <img class="card-img-top" src="49824.php" alt="A simple MacBook Air with a glass of water next to it">
  <div class="card-body">
    <h5 class="card-title">Mac users are lucky!</h5>
    <p class="card-text">Our application designed for Mac users allows them to have a more stable version of our interface as well as a more pleasant design and unbeatable speed.<br>
    More secure, easier, and more efficient.</p>
    <div class="text-center mt-4">
    <a href="#" class="btn btn-primary">Download</a>
    </div>
</div>
</div>
        </div>
        
        <div class="col-sm-4">
            <div class="card" style="width: 18rem;">
  <img class="card-img-top" src="1114478.php" alt="A brown-haired woman smiling while being on the phone">
  <div class="card-body">
    <h5 class="card-title">Broadcom Wi-Fi</h5>
    <p class="card-text">With our technology 'Broadcom Wi-Fi', calling your friends has never been so easy before!<br>You can call them outside of your house, even when there's no Wi-Fi around you.<br><br></p>
    <div class="text-center mt-4">
    <a href="#" class="btn btn-primary">Try</a>
    </div>
</div>
</div>
        </div>
        <div class="col-sm-4">
            <div class="card" style="width: 18rem;">
  <img class="card-img-top" src="59843.php" alt="A group of friends discussing in a classroom">
  <div class="card-body">
    <h5 class="card-title">Keep contact with your friends.</h5>
    <p class="card-text">A good reason for using Messenger is that you can stay in contact with anyone you meet in life.<br>Add your friends by adding their unique IDs.</p>
    <div class="text-center mt-4">
    <a href="#" class="btn btn-primary">Get an ID</a>
    </div>
</div>
</div>
        </div>
                </div>
                </div>
                </div>
        </div>

这是 css 的预览（我将发布我的所有样式表，因为我不知道它是如何工作的）：

@import url('https://fonts.googleapis.com/css2?family=Roboto:wght@300;400;500;700&display=swap');

* {
  -webkit-font-smoothing: antialiased;
  -moz-osx-font-smoothing: grayscale;
  -webkit-user-select: none;
  -moz-user-select: none;
  -ms-user-select: none;
  user-select: none;
}

h1,
h2,
h3,
h4,
h5,
h6 {
    font-weight:700 !important;
}

p {
    font-weight:300;
}

body {
    background:#fff !important;
    font-family:'Roboto', sans-serif !important;
}

select {
    padding:0 30px 0 10px !important;
    -webkit-padding-end: 30px !important;
    -webkit-padding-start: 10px !important;
}

span.navbar-text {
    font-size:9pt;
    color:rgba(0,0,0,.55);
    font-weight:400;
}

p.desc {
    color:rgba(0,0,0,.55);
}

.dropdown-menu {
    background:#fff !important;
    border-radius:0 !important;
    font-weight:300 !important;
    color:rgba(0,0,0,.55) !important;
    max-width:50%;
    position:absolute;
}

a.dropdown-item {
    font-weight:300 !important;
    color:rgba(0,0,0,.55) !important;
}

.navbar {
    border-bottom:1px solid rgba(0,0,0,0.04);
    background:#fff;
}

.jumbotron {
    background:rgba(255,255,255,0);
    color:#000;
    margin:0 auto;
    border-radius:0;
}

.px-11 {
    font-size:11pt;
}

.pd-20 {
    padding-top:20px;
}

img.brand {
    transition:transform 0.5s ease;
}

img.brand:hover {
    transform: scale(1.06);
}

ul.navbar-nav {
    margin-left:10px;
}

a {
    letter-spacing:0.1px;
    font-size:11pt;
    font-weight:300;
    color: rgba(0, 0, 0, .55);
}

li.active > a {
    font-weight:500;
}

li {
    padding-right:10px;
}

a:hover {
    color:#000;
    text-decoration:underline;
}

.brand-text {
    font-size:11pt;
    font-weight:300;
    color:#000;
    position:relative;
}

h1 {
    color:#000;
}

.form-control {
    border-radius:6px;
    background:#fff;
    color:rgba(0,0,0,.55);
    font-size:10pt;
    font-weight:300;
    border:1px solid rgba(0,0,0,0.04);
    width:50%;
    letter-spacing:-0.1px;
    padding:20px 15px;
}

.form-control:focus {
  border:1px solid rgba(0,0,0,0.08);
  box-shadow:none;
}

nav ul li a,
nav ul li a:after,
nav ul li a:before {
  transition: all 0.2s ease;
}

label {
    font-weight:300;
    font-size:11pt;
}

.btn {
    background:#0078FF;
    color:#fff;
    font-size:10pt;
    border-radius:3px;
    font-weight:500;
}

.btn:hover {
    color:#fff;
}

.smart-scroll{
  position: fixed;
  top: 0;
  right: 0;
  left: 0;
  z-index: 1030;
}
.scrolled-down{
   transform:translateY(-100%); transition: all 0.3s ease-in-out;
}
.scrolled-up{
   transform:translateY(0); transition: all 0.3s ease-in-out;
}

.card-title {
    font-size:16pt !important;
    font-weight:700 !important;
}

.card-text {
    font-size:9pt;
    font-weight:400;
    color:rgba(0,0,0,.55);
}

.card {
    border-radius:10px;
    border:0;
    box-shadow:0px 0px 20px rgba(0,0,0,.07) !important;
}

.features {
    margin-top:75px !important;
    margin-bottom:50px;
}

下面是我的问题的预览：

Answer 1

从有类卡片的div中移除宽度

<div class="card" style="width: 18rem;">

到：

<div class="card">

为你的 col div 或你的 card div 使用 padding 和 margin => https://getbootstrap.com/docs/4.1/utilities/spacing/

Answer 2

在这种情况下，只需使用

<center>
your code you want to center
</center>

95% 的时间都在工作