html{
    color-scheme: light;
}

:root{
    --page-bg: hsl(0 0% 95%);
    --page-text: hsl(0 0% 10%);

    --calc-bg: hsl(0 0% 15%);
    --display-bg: hsl(0 0% 20%);

    --btn-bg: hsl(0 0% 30%);
    --btn-bg-hover: hsl(0 0% 40%);
    --btn-bg-active: hsl(0 0% 50%);

    --operator-bg: hsl(35 100% 55%);
    --operator-bg-hover: hsl(35 100% 65%);
    --operator-bg-active: hsl(35 100% 75%);

    --toggle-bg: hsl(0 0% 100% / 0.75);
    --toggle-bg-hover: hsl(0 0% 100% / 0.9);
    --toggle-border: hsl(0 0% 10% / 0.2);
}

html[data-theme="dark"]{
    color-scheme: dark;

    --page-bg: hsl(0 0% 10%);
    --page-text: hsl(0 0% 96%);

    --calc-bg: hsl(0 0% 16%);
    --display-bg: hsl(0 0% 22%);

    --btn-bg: hsl(0 0% 32%);
    --btn-bg-hover: hsl(0 0% 40%);
    --btn-bg-active: hsl(0 0% 48%);

    --toggle-bg: hsl(0 0% 18% / 0.75);
    --toggle-bg-hover: hsl(0 0% 18% / 0.9);
    --toggle-border: hsl(0 0% 100% / 0.2);
}

body{
    margin: 0;
    display: flex;
    flex-direction: column;
    justify-content: center;
    align-items: center;
    height: 100vh;
    gap: 20px;
    background-color: var(--page-bg);
    color: var(--page-text);
}

#header{
    text-align: center;
    font-family: 'Segoe UI', Tahoma, Geneva, Verdana, sans-serif;
}

#header h1{
    margin: 0;
}

#header h4{
    margin: 8px 0 0 0;
    font-weight: 600;
}

#theme-toggle{
    position: fixed;
    top: 16px;
    left: 16px;
    z-index: 10;
    padding: 10px 12px;
    border-radius: 999px;
    border: 1px solid var(--toggle-border);
    background: var(--toggle-bg);
    color: var(--page-text);
    font: inherit;
    font-weight: 600;
    cursor: pointer;
    backdrop-filter: blur(8px);
}

#theme-toggle:hover{
    background: var(--toggle-bg-hover);
}

#theme-toggle:active{
    transform: translateY(1px);
}

#calculator{
    font-family: Arial, sans-serif;
    background-color: var(--calc-bg);
    border-radius: 15px;
    max-width: 500px;
    overflow: hidden;
}

#display{
    width: 100%;
    padding: 20px;
    font-size: 5rem;
    text-align: left;
    border: none;
    background-color: var(--display-bg);
    color: white;
}

#keys{
    display: grid;
    grid-template-columns: repeat(4, 1fr);
    gap: 10px;
    padding: 25px;
}

#keys button{
    width: 100px;
    height: 100px;
    border-radius: 50px;
    border: none;
    background-color: var(--btn-bg);
    color: white;
    font-size: 3rem;
    font-weight: bold;
    cursor: pointer;
}

#keys button:hover{
    background-color: var(--btn-bg-hover);
}

#keys button:active{
    background-color: var(--btn-bg-active);
}

#keys .operator-btn{
    background-color: var(--operator-bg);
}

#keys .operator-btn:hover{
    background-color: var(--operator-bg-hover);
}

#keys .operator-btn:active{
    background-color: var(--operator-bg-active);
}