▶ 运行代码@mixin flex-center {
    display: flex;
    justify-content: center;
    align-items: center;
}

.card {
    @include flex-center;
    padding: 20px;
}

%button-base {
    padding: 10px 20px;
    border-radius: 4px;
}

.btn-primary {
    @extend %button-base;
    background: blue;
}