:root {
    --bg-color: #f8fafc;
    --container-bg: #ffffff;
    --text-primary: #0f172a;
    --text-secondary: #475569;
    --border-color: #e2e8f0;
    --primary-color: #007bff;
    --primary-hover: #0056b3;
    --error-color: #ef4444;
    --success-color: #22c55e;
    --warning-color: #f59e0b;
    --transition-speed: 0.2s;
    
    --shadow-sm: 0 1px 2px 0 rgb(0 0 0 / 0.05);
    --shadow-md: 0 4px 6px -1px rgb(0 0 0 / 0.1), 0 2px 4px -2px rgb(0 0 0 / 0.1);
    --shadow-lg: 0 10px 15px -3px rgb(0 0 0 / 0.1), 0 4px 6px -4px rgb(0 0 0 / 0.1);

    /* Typography Scale */
    --font-sans: 'Inter', system-ui, -apple-system, sans-serif;
    --text-xs: 0.75rem;     /* 12px */
    --text-sm: 0.875rem;    /* 14px */
    --text-base: 1rem;      /* 16px */
    --text-lg: 1.125rem;    /* 18px */
    --text-xl: 1.25rem;     /* 20px */
    --text-2xl: 1.5rem;     /* 24px */
    --text-3xl: 1.875rem;   /* 30px */
    --text-4xl: 2.25rem;    /* 36px */
    
    --leading-tight: 1.25;
    --leading-normal: 1.5;
    --leading-relaxed: 1.625;
    
    --font-normal: 400;
    --font-medium: 500;
    --font-semibold: 600;
    --font-bold: 700;
}

[data-theme='dark'] {
    --bg-color: #0f172a;
    --container-bg: #1e293b;
    --text-primary: #f8fafc;
    --text-secondary: #94a3b8;
    --border-color: #334155;
    --primary-color: #3b82f6;
    --primary-hover: #60a5fa;
    --error-color: #f87171;
    --success-color: #4ade80;
    --warning-color: #fbbf24;

    --shadow-sm: 0 1px 2px 0 rgb(0 0 0 / 0.3);
    --shadow-md: 0 4px 6px -1px rgb(0 0 0 / 0.5), 0 2px 4px -2px rgb(0 0 0 / 0.5);
    --shadow-lg: 0 10px 15px -3px rgb(0 0 0 / 0.6), 0 4px 6px -4px rgb(0 0 0 / 0.6);
}

body {
    font-family: Arial, sans-serif;
    margin: 0;
    background-color: var(--bg-color);
    color: var(--text-primary);
    transition: background-color var(--transition-speed), color var(--transition-speed);
}

.signup-container, .login-container {
    background: var(--container-bg);
    padding: 2rem;
    border-radius: 8px;
    box-shadow: var(--shadow-md);
    width: 100%;
    max-width: 400px;
    transition: background-color var(--transition-speed), box-shadow var(--transition-speed);
}

h1 {
    text-align: center;
    color: var(--text-primary);
}

.form-group {
    margin-bottom: 1rem;
}

label {
    display: block;
    margin-bottom: 0.5rem;
    color: var(--text-secondary);
}

input {
    width: 100%;
    padding: 0.5rem;
    border: 1px solid var(--border-color);
    border-radius: 4px;
    box-sizing: border-box; /* Important for responsive width */
    background-color: var(--container-bg);
    color: var(--text-primary);
    transition: border-color var(--transition-speed), background-color var(--transition-speed), color var(--transition-speed);
}

form button[type="submit"] {
    padding: 0.75rem;
    background-color: var(--primary-color);
    color: white;
    border: none;
    border-radius: 4px;
    cursor: pointer;
    font-size: 1rem;
    transition: background-color var(--transition-speed);
}

form button[type="submit"]:hover {
    background-color: var(--primary-hover);
}

p {
    text-align: center;
    margin-top: 1rem;
    color: var(--text-secondary);
}

input.is-invalid {
    border-color: var(--error-color);
}

input.is-valid {
    border-color: var(--success-color);
}

.invalid-feedback {
    display: none;
    width: 100%;
    margin-top: 0.25rem;
    font-size: 0.875rem;
    color: var(--error-color);
}

input.is-invalid ~ .invalid-feedback,
.invalid-feedback:not(:empty) {
    display: block;
}

form button[type="submit"]:disabled {
    background-color: var(--border-color);
    cursor: not-allowed;
}

.loading {
    opacity: 0.7;
    pointer-events: none;
}

/* Theme Toggle Styling — sits in the header flex row, not absolutely positioned. */
#theme-toggle {
    width: auto;
    padding: 0.5rem;
    background: transparent;
    border: 1px solid var(--border-color);
    border-radius: 50%;
    color: var(--text-primary);
    font-size: 1.2rem;
    line-height: 1;
    display: flex;
    align-items: center;
    justify-content: center;
}

nav button {
    width: auto;
}

#theme-toggle:hover {
    background: rgba(0, 0, 0, 0.05);
}

[data-theme='dark'] #theme-toggle:hover {
    background: rgba(255, 255, 255, 0.05);
}

/* Table Sorting Styles */
th.sortable {
    cursor: pointer;
    position: relative;
    padding-right: 1.5rem !important;
}

th.sortable:after {
    content: '↕';
    position: absolute;
    right: 0.5rem;
    opacity: 0.3;
}

th.sortable.sort-asc:after {
    content: '↑';
    opacity: 1;
}

th.sortable.sort-desc:after {
    content: '↓';
    opacity: 1;
}

@media (max-width: 480px) {
    .signup-container, .login-container {
        padding: 1rem;
        margin: 1rem;
    }
}