style.css

*{
    margin: 0;
    padding: 0;
    box-sizing: border-box;
}

:root{
    --primary-bg: #140b27;
    --pure-white: #ffffff;
    --grey-clr: #8080809b;
    --light-blue: #cbe2f1a5;
    --segoe-font: 'Segoe UI', Tahoma, Geneva, Verdana, sans-serif;
}

body{
    display: flex;
    align-items: center;
    justify-content: center;
    height: 100vh;
    background-color: var(--primary-bg);
}

.calculator-big-screen{
    width: 30%;
    padding: 20px 10px;
    border-radius: 15px;
    background-color: var(--pure-white);
    box-shadow: 0 0 10px var(--grey-clr);
}

.number-box{
    width: 95%;
    height: 100px;
    display: block;
    border-radius: 15px;
    margin: 20px auto;
    border: none;
    box-shadow: 0 0 7px var(--grey-clr);
    font-size: 48px;
    text-align: right;
    font-family: var(--segoe-font);
    font-weight: bold;
    padding-right: 15px;
    pointer-events: none;
}

.calculator-container{
    display: flex;
    margin: 0 20px;
    justify-content: space-between;
    align-items: center;
    gap: 30px;
    padding: 10px 0;
}

.btn,.clear-btn,.backsp-btn,.equal-to{
    width: 100%;
    height: 50px;
    border: none;
    outline: none;
    border-radius: 10px;
    background-color: #cbe2f1a5;
    box-shadow: 0 0 2px var(--grey-clr);
    cursor: pointer;
    font-size: 24px;
    transition: 0.25s;
    font-weight: bold;
    font-family: var(--segoe-font);
}

.btn:hover{
    opacity: 0.7;
}

.operation-btn{
    background-color: #4B99F7;
    color: var(--pure-white);
}

.equal-to{
    background-color: #FC3C64;
    color: var(--pure-white);
}

@media (min-width: 768px) and (max-width: 1024px){
    .calculator-big-screen{
        width: 60%;
    }
}

@media (max-width: 767px){
    .calculator-big-screen{
        width: 90%;
    }
}