:root {
    --calc-bg-color: #3F3F3F;
    --display-bg-color: #1a1a1a;
    --display-font-color: #ccc;

    --numpad-btn-color: rgb(233, 233, 233);
    --numpad-active-btn-color: rgb(151, 151, 151);

    --numpad-bg-btn-color: gray;
    --numpad-active-bg-btn-color: #424242;

    --operation-bg-btn-color: rgb(4, 168, 4);
    --operation-active-bg-btn-color: rgb(3, 110, 3);

    --memory-bg-btn-color: rgb(4, 168, 4);
    --memory-active-bg-btn-color: rgb(3, 110, 3);

    --bevel-radius: 10px;
}

html {
    margin:0;
    padding:0;
}

body * {
    font-size: 20px;
    font-family: 'Orbitron', Arial, sans-serif;
}

button {
    cursor: pointer;
    outline: none;
}

input {
    outline: none;
}

/*
------------------------------
BUTTONS
------------------------------
*/

.btn-group {
    margin: 20px 6px;
    display: flex;
    flex-direction: row;
    justify-content: space-between;
}

.btn-group:last-child {
    margin-bottom: 5px;
}

.btn-group button {
    padding: 10px;
    width: 60px;
    height: 60px;
    color: var(--numpad-btn-color);
    background-color: var(--numpad-bg-btn-color);
    font-size: inherit;
    border: 2px solid black;
    border-radius: var(--bevel-radius);
    box-shadow: 0px 3px 5px black;
}
.btn-group button:hover,
.btn-group button:active,
.btn-group button:focus {
    color: var(--numpad-active-btn-color);
    box-shadow: none;
    background-color: var(--numpad-active-bg-btn-color);
}

button.operation {
    background-color: var(--operation-bg-btn-color);
}

button.operation.active {
    color: var(--numpad-active-btn-color);
    background-color: var(--operation-active-bg-btn-color);
}

button.operation:hover,
button.operation:active,
button.operation:focus {
    color: var(--numpad-active-btn-color);
    background-color: rgb(3, 110, 3);

}

button.memory {
    color: var(--memory-btn-color);
}

/*
------------------------------
CALCULATOR
------------------------------
*/
.calculator {
    position: relative;
    display: flex;
    flex-direction: column;
    justify-content: center;
    width: 100%;
    max-width: 420px;
    min-width: 280px;
    height: auto;
    margin: auto;
    margin-top: 50px;
    background-color: var(--calc-bg-color);
    border: 3px solid black;
    border-radius: var(--bevel-radius);
    box-shadow: 0px 5px 5px rgb(39, 39, 39);
}

/*
------------------------------
DISPLAY
------------------------------
*/
.display {
    position: relative;
    width: auto;
    height: auto;
    border: 1px solid var(--calc-bg-color);
    box-sizing: border-box;
    margin: 25px 10px 10px 10px;

}

.display-group {
    position: relative;
    display: flex;
    flex-direction: row;
    justify-content: space-evenly;
    box-sizing: inherit;
}

.clear-btn {
    position: absolute;
    width: 60px;
    height: 100%;
    top: 0;
    left: 0;
    color: var(--numpad-btn-color);
    background-color: var(--operation-bg-btn-color);
    border: 5px solid black;
    border-right: none;
    border-radius: var(--bevel-radius) 0 0 var(--bevel-radius);
    font-size: inherit;
}

.clear-btn:active,
.clear-btn:hover {
    color: var(--numpad-active-btn-color);
    background-color:var(--operation-active-bg-btn-color);
}

.memory-input {
    display: block;
    margin-top: 10px;
    width: 100%;
    font-size: 65%;
    color: var(--display-font-color);
    background-color: black;
    border: 1.5px solid rgba(0, 0, 0, 0);
    padding: 5px 10px;
    border-radius: var(--bevel-radius);
    box-sizing: inherit;
}

.display-input {
    display: block;
    padding: 5px 10px;
    width: 100%;
    color: var(--display-font-color);
    text-align: right;
    font-size: 160%;
    border: 5px solid black;
    background-color: var(--display-bg-color);
    border-radius: var(--bevel-radius);
    outline: none;
}

/*
------------------------------
PAD
------------------------------
*/

.pad {
    display: flex;
    flex-direction: column;
    justify-content: space-evenly;
    padding: 5px;
}

@media (max-width: 600px) {
    .calculator {
        max-width: 85%;
    }
}