@import url('https://fonts.googleapis.com/css?family=Muli&display=swap');

* {
    box-sizing: border-box;
}

/* 声明全局变量 */
:root {
    --line-border-fill: #3498db;
    --line-border-empty: #e0e0e0;
}

body {
    font-family: 'Muli', sans-serif;
    background-color: rgb(236, 236, 236);
    height: 95vh;
}

#title {
    text-align: center;
    padding-top: 5vh;
}

.container {
    display: block;
}

.progress-container {
    display: flex;                  /* 弹性容器，影响子元素排列 */
    width: 300px;
    justify-content: space-between; /* 控制子元素间距 */
    margin-top: 40vh;

    /* 常用方法，使元素居中 */
    margin-left: auto;
    margin-right: auto;

    /* 本例中是为了提供给进度条用于定位 */
    position: relative;
}

/* 伪元素，生成一个进度条底板 */
.progress-container:before {
    content: '';
    background-color: var(--line-border-empty);

    /* 基于absolute的垂直居中方法 */
    position: absolute;
    top: 50%;
    transform: translateY(-50%);

    height: 5px;
    width: 100%;
    z-index: -1;    /* 会被更高z-index的元素覆盖 */
}

.progress {
    background-color: var(--line-border-fill);

    /* 基于absolute的垂直居中方法 */
    position: absolute;
    top: 50%;
    transform: translateY(-50%);

    height: 5px;
    width: 0%;
    z-index: -1;
    transition: 0.4s ease-in-out;
}

.circle {
    border-radius: 50%;
    height: 25px;
    width: 25px;

    /* 子元素居中 */
    display: flex;
    align-items: center;
    justify-content: center;

    background-color: white;
    color: #999;
    border: 3px solid var(--line-border-empty);
    transition: all 0.4s ease-in-out;
}

.circle.active {
    border-color: var(--line-border-fill);
    transition: all 0.4s ease-in-out;
}

.btn-container {
    /* 子元素居中 */
    display: flex;
    align-items: center;
    justify-content: center;
}

/* 按钮样式 */
.btn {
    background-color: var(--line-border-fill);
    color: #fff;
    border: none;
    border-radius: 6px;
    cursor: pointer;
    font-family: inherit;

    /* 按钮间距 */
    margin-top: 3vh;
    margin-left: 1vw;
    margin-right: 1vw;

    padding: 8px 30px;      /* 按钮大小 */
    font-size: 14px;
}

.btn:disabled {
    background-color: var(--line-border-empty);
    cursor: not-allowed;
}