:root {
    --strong-cyan: hsl(172, 67%, 45%);
    --very-dark-cyan: hsl(183, 100%, 15%);
    --dark-grayish-cyan: hsl(186, 14%, 43%);
    --grayish-cyan: hsl(184, 14%, 56%);
    --light-grayish-cyan: hsl(185, 41%, 84%);
    --very-light-grayish-cyan: hsl(189, 41%, 97%);
    --white: hsl(0, 0%, 100%);
    --error-color: hsl(13, 70%, 60%);
}

* {
    margin: 0;
    padding: 0;
    box-sizing: border-box;
    font-family: 'Space Mono', monospace;
}

body {
    background-color: var(--light-grayish-cyan);
    display: flex;
    justify-content: center;
    align-items: center;
    min-height: 100vh;
    padding: 2rem;
}

.container {
    width: 100%;
    max-width: 800px;
}

header h1 {
    color: var(--very-dark-cyan);
    text-align: center;
    letter-spacing: 0.5rem;
    margin-bottom: 2rem;
    line-height: 1.2;
    font-weight: 700;
    font-size: 1.5rem;
}

.calculator {
    background-color: var(--white);
    border-radius: 1rem;
    padding: 2rem;
    display: flex;
    gap: 2rem;
}

.input-section {
    flex: 1;
    display: flex;
    flex-direction: column;
    gap: 2rem;
}

.input-group {
    display: flex;
    flex-direction: column;
    gap: 0.5rem;
}

label {
    color: var(--dark-grayish-cyan);
    font-weight: 700;
}

.input-field {
    position: relative;
    display: flex;
    align-items: center;
}

.input-field span {
    position: absolute;
    left: 1rem;
    color: var(--grayish-cyan);
    font-weight: 700;
}

input {
    width: 100%;
    padding: 0.5rem 1rem;
    padding-left: 2.5rem;
    font-size: 1.2rem;
    font-weight: 700;
    color: var(--very-dark-cyan);
    background-color: var(--very-light-grayish-cyan);
    border: none;
    border-radius: 0.3rem;
    text-align: right;
}

input:focus {
    outline: 2px solid var(--strong-cyan);
}

.tip-buttons {
    display: grid;
    grid-template-columns: repeat(3, 1fr);
    gap: 1rem;
}

.tip-btn {
    background-color: var(--very-dark-cyan);
    color: var(--white);
    border: none;
    border-radius: 0.3rem;
    padding: 0.5rem;
    font-size: 1.2rem;
    font-weight: 700;
    cursor: pointer;
    transition: all 0.2s;
}

.tip-btn:hover {
    background-color: var(--light-grayish-cyan);
    color: var(--very-dark-cyan);
}

.tip-btn.active {
    background-color: var(--strong-cyan);
    color: var(--very-dark-cyan);
}

.custom-tip {
    padding-left: 1rem !important;
    text-align: center;
}

.error-message {
    color: var(--error-color);
    font-weight: 700;
    align-self: flex-end;
    display: none;
}

.error {
    outline: 2px solid var(--error-color) !important;
}

.output-section {
    flex: 1;
    background-color: var(--very-dark-cyan);
    border-radius: 0.8rem;
    padding: 2rem;
    display: flex;
    flex-direction: column;
    justify-content: space-between;
}

.result {
    display: flex;
    flex-direction: column;
    gap: 2rem;
}

.result-item {
    display: flex;
    justify-content: space-between;
    align-items: center;
}

.result-item h2 {
    color: var(--white);
    font-size: 1rem;
}

.result-item span {
    color: var(--grayish-cyan);
    font-size: 0.8rem;
}

.amount {
    color: var(--strong-cyan);
    font-size: 2rem;
    font-weight: 700;
}

.reset-btn {
    width: 100%;
    padding: 0.5rem;
    background-color: var(--strong-cyan);
    color: var(--very-dark-cyan);
    border: none;
    border-radius: 0.3rem;
    font-size: 1.2rem;
    font-weight: 700;
    text-transform: uppercase;
    cursor: pointer;
    transition: all 0.2s;
}

.reset-btn:hover {
    background-color: var(--light-grayish-cyan);
}

.reset-btn:disabled {
    background-color: hsl(183, 79%, 24%);
    color: hsl(183, 100%, 15%);
    cursor: not-allowed;
    opacity: 0.3;
}

@media (max-width: 768px) {
    .calculator {
        flex-direction: column;
    }
    
    .tip-buttons {
        grid-template-columns: repeat(2, 1fr);
    }
}

.attribution {
    text-align: center;
    font-size: 0.8rem;
    color: var(--grayish-cyan);
    margin-top: 1.5rem;
    font-weight: 500;
}

.attribution a {
    color: var(--strong-cyan);
    text-decoration: none;
    font-weight: 700;
}

.attribution a:hover {
    text-decoration: underline;
}