index.css

:root {
    --color-primary: #e6e6e6;
    --color-result-background: #222222;
    --color-button-numeric: #595959;
    --color-button-operation-primary: #f7a239;
    --color-button-operation-secondary: #383838;
    --color-button-operation-primary-clicked: #c5802b;
    --color-button-operation-secondary-clicked: #595959;
    --color-button-numeric-clicked: #9b9b9b;
    --color-button-primary-border: #222222;
    --width-button: 24.75%;
    --width-button-double: 49.75%;
}

body {
    min-height: 630px;
    padding: 0;
    margin: 0;
    box-sizing: border-box;
}

.body {
    height: 100vh;
    display: flex;
    align-items: center;
    justify-content: center;
}

.calculator {
    display: flex;
    flex-direction: column;  
    width: 460px;
    background-color: var(--color-result-background);
}

.results {
    font-family: monospace;
    padding: 50px 10px 10px 0px;
    font-size: 70px;
    color: var(--color-primary);
    background-color: var(--color-result-background);
    text-align: right;
}

.buttons-row {
    display: flex;
    margin-bottom: 0.25%;
    justify-content: space-between;
}

.buttons-row:last-of-type {
    margin-bottom: 0;
}

.button {
    font-size: 30px;
    background-color: var(--color-button-numeric);
    color: var(--color-primary);
    width: var(--width-button);
    height: 95px;
    border: none;
    user-select: none;
}

.primary-op {
    font-size: 35px;
    background-color: var(--color-button-operation-primary);
}

.secondary-op {
    background-color: var(--color-button-operation-secondary);
}

.secondary-op--clicked {
    background-color: var(--color-button-operation-secondary-clicked);
}

.primary-op--clicked {
    background-color: var(--color-button-operation-primary-clicked);
}

.primary-op--selected {
    border: var(--color-button-primary-border) 4px solid;
    border-right: none;
}

.numeric--clicked {
    background-color: var(--color-button-numeric-clicked);
}

.double {
    width: var(--width-button-double);
}