:root {
    --gradient-invert-color: linear-gradient(0deg,rgb(83,170,223), rgb(37, 47,191));
    --gradient-color: linear-gradient(180deg, rgb(83,170,223), rgb(37, 47,191));
    --gradient-90deg-color: linear-gradient(90deg, rgb(83,170,223), rgb(37, 47,191));
    --gradient-135deg-color: linear-gradient(135deg, rgb(83,170,223), rgb(37, 47,191));
    --gradient-315deg-color: linear-gradient(315deg, rgb(83,170,223), rgb(37, 47, 191));
    --gradient2: linear-gradient(144deg,#AF40FF, #5B42F3 50%,#00DDEB);
    --primary-color: rgb(79,183,220);
    --bg-color: #101630;
    --text-color: rgba(255, 255, 255, .6);

    --border-radius: 1.4rem;
    --box-shadow-item: -.1rem -.1rem 2rem rgba(0, 0, 0, .2);

    /* Color */
    --box-shadow: 0 .4rem .6rem var(--gray2);

    --header-height: 8rem;
}

* {
    box-sizing: inherit;
}
html {
    /* 1rem = 10px */
    font-size: 62.5%;
    line-height: 1.6rem;
    font-family: 'Inter', sans-serif;
    box-sizing: border-box;
    scroll-behavior: smooth;
}
body {
    margin: 0;
}
h1, h2, h3, h4, h5, h6, ul, p, button {
    padding: 0;
    margin: 0;
    color: white;
}
h1 {
    /* h1: heading SEO */
    --size: 12rem;
    font-size: var(--size);
    line-height: var(--size);
}
h2 {
    /* h2: sub heading */
    --size: 5rem;
    font-size: var(--size);
    line-height: calc(var(--size) * 1.5);
}
h3 {
    /* h3: session title */
    --size: 4rem;
    font-size: var(--size);
    line-height: calc(var(--size) * 1.5);
    text-transform: uppercase;
    font-weight: 600;
    text-align: center;
}
h4 {
    /* h4: title item*/
    --size: 3rem;
    font-size: var(--size);
    line-height: calc(var(--size) * 1.5);
    text-transform: uppercase;
    font-weight: 500;
}
h5, h6, p {
    /* h5: text bold & header bar*/
    /* h6: text regular */
    /* p: paragraph */
    --size: 2rem;
    font-size: var(--size);
    line-height: calc(var(--size) * 1.5);
}
h5 {
    font-weight: 500;
}
h6 {
    font-weight: 300;
}
p {
    color: var(--text-color);
    text-align: justify;
}

/* Margin */
.mt-4 {
    margin-top: .4rem !important;
}
.mt-8 {
    margin-top: .8rem !important;
}
.mt-12 {
    margin-top: 1.2rem !important;
}
.mt-16 {
    margin-top: 1.6rem !important;
}
.mt-20 {
    margin-top: 2rem !important;
}
.mt-24 {
    margin-top: 2.4rem !important;
}
.mt-32 {
    margin-top: 3.2rem !important;
}

.ml-4 {
    margin-left: .4rem !important;
}
.ml-8 {
    margin-left: .8rem !important;
}
.ml-12 {
    margin-left: 1.2rem !important;
}
.ml-16 {
    margin-left: 1.6rem !important;
}
.ml-20 {
    margin-left: 2rem !important;
}
.ml-24 {
    margin-left: 2.4rem !important;
}
.ml-32 {
    margin-left: 3.2rem !important;
}

/* Padding */
.pl-32 {
    padding-left: 3.2rem !important;
}
.pr-32 {
    padding-right: 3.2rem !important;
}

/* !No Use */
.no-hover {
    cursor: default;
}
.no-select {
    user-select: none;
    -webkit-user-select: none;
}
.black-white {
    filter: grayscale(100%);
    opacity: .2;
}
/* Animation */
@keyframes fadeIn {
    from {
        opacity: 0;
    }
    to {
        opacity: 1;
    }
}

@keyframes zoomGrowth {
    from {
        transform: scale(0);
    }
    to {
        transform: 1;
    }
}

@keyframes rotation {
    from {
        transform: rotate(0);
    } to {
        transform: rotate(360deg);
    }
}

/* Button */
.btn {
    display: flex; 
    align-items: center;
    justify-content: center;
    text-decoration: none;
    user-select: none;
    -webkit-user-select: none;
    background-image: var(--gradient-color);
    border-radius: var(--border-radius);
    padding: .3rem;
    /* padding ở đây xem như là độ dày của border sau khi hover */
}
.btn__text {
    color: white;
    border-radius: var(--border-radius);
    background-image: var(--gradient-color);
    width: 100%;
    padding: 2.4rem 0;
    text-align: center;
}
.btn:hover .btn__text {
    background-image: var(--gradient-invert-color);
}


#scrollToTop {
    display: none;
    position: fixed; /* Fixed/sticky position */
    bottom: 20px; /* Place the button at the bottom of the page */
    right: 30px; /* Place the button 30px from the right */
    z-index: 99; /* Make sure it does not overlap */
    border: none; /* Remove borders */
    outline: none; /* Remove outline */
    background: rgba(79,183,220, 0.2);
    box-shadow: var(--box-shadow);
    color: white; /* Text color */
    cursor: pointer; /* Add a mouse pointer on hover */
    padding: 1rem; /* Some padding */
    border-radius: 10px; /* Rounded corners */
    font-size: 3rem; /* Increase font size */
}
#scrollToTop a {
    color: white; /* Text color */
    text-decoration: none;
}
#scrollToTop:hover {
    background-color: var(--primary-color); /* Add a dark-grey background on hover */
}