:root {
    --cbt-primary: #2954E5;
    --cbt-primary-dark: #1E3FB8;
    --cbt-ink: #0F1729;
    --cbt-slate: #57607A;
    --cbt-bg: #F5F7FB;
    --cbt-surface: #FFFFFF;
    --cbt-border: #E4E8F1;
    --cbt-success: #17A673;
    --cbt-warning: #F5A623;
    --cbt-danger: #E5484D;
    --cbt-radius: 12px;
    --cbt-sidebar-w: 264px;
}

[data-theme="dark"] {
    --cbt-ink: #F2F4F8;
    --cbt-slate: #A9B2C7;
    --cbt-bg: #0B1120;
    --cbt-surface: #131B2E;
    --cbt-border: #232D44;
}

body {
    background: var(--cbt-bg);
    color: var(--cbt-ink);
    font-family: 'Inter', system-ui, -apple-system, sans-serif;
    font-size: 0.925rem;
}

/* ---------- Layout shell ---------- */
.cbt-sidebar {
    width: var(--cbt-sidebar-w);
    min-height: 100vh;
    background: var(--cbt-surface);
    border-right: 1px solid var(--cbt-border);
    position: fixed;
    top: 0; left: 0; bottom: 0;
    overflow-y: auto;
    z-index: 1030;
    transition: transform .2s ease;
}
.cbt-main {
    margin-left: var(--cbt-sidebar-w);
    min-height: 100vh;
}
.cbt-topbar {
    background: var(--cbt-surface);
    border-bottom: 1px solid var(--cbt-border);
    position: sticky;
    top: 0;
    z-index: 1020;
}
@media (max-width: 991.98px) {
    .cbt-sidebar { transform: translateX(-100%); }
    .cbt-sidebar.show { transform: translateX(0); }
    .cbt-main { margin-left: 0; }
}

/* ---------- Brand ---------- */
.cbt-brand {
    font-weight: 700;
    letter-spacing: -0.02em;
    color: var(--cbt-ink);
}
.cbt-brand .accent { color: var(--cbt-primary); }

/* ---------- Nav ---------- */
.cbt-nav-link {
    display: flex;
    align-items: center;
    gap: .65rem;
    padding: .55rem .9rem;
    border-radius: 10px;
    color: var(--cbt-slate);
    font-weight: 500;
    font-size: .875rem;
    text-decoration: none;
    margin: 2px 10px;
}
.cbt-nav-link i { font-size: 1.05rem; width: 20px; text-align: center; }
.cbt-nav-link:hover { background: rgba(41,84,229,.08); color: var(--cbt-primary); }
.cbt-nav-link.active { background: var(--cbt-primary); color: #fff; }
.cbt-nav-section {
    font-size: .7rem;
    text-transform: uppercase;
    letter-spacing: .08em;
    color: var(--cbt-slate);
    padding: 1rem 1.15rem .3rem;
    font-weight: 600;
}

/* ---------- Cards ---------- */
.cbt-card {
    background: var(--cbt-surface);
    border: 1px solid var(--cbt-border);
    border-radius: var(--cbt-radius);
}
.cbt-stat-card {
    background: var(--cbt-surface);
    border: 1px solid var(--cbt-border);
    border-radius: var(--cbt-radius);
    padding: 1.1rem 1.25rem;
    display: flex;
    align-items: center;
    gap: 1rem;
}
.cbt-stat-icon {
    width: 46px; height: 46px;
    border-radius: 10px;
    display: flex; align-items: center; justify-content: center;
    font-size: 1.2rem;
    flex-shrink: 0;
}
.cbt-stat-value { font-size: 1.5rem; font-weight: 700; line-height: 1.1; }
.cbt-stat-label { font-size: .78rem; color: var(--cbt-slate); font-weight: 500; }

.cbt-badge-soft-success { background: rgba(23,166,115,.12); color: var(--cbt-success); }
.cbt-badge-soft-warning { background: rgba(245,166,35,.12); color: var(--cbt-warning); }
.cbt-badge-soft-danger  { background: rgba(229,72,77,.12); color: var(--cbt-danger); }
.cbt-badge-soft-primary { background: rgba(41,84,229,.1); color: var(--cbt-primary); }

/* ---------- Auth pages ---------- */
.cbt-auth-wrap {
    min-height: 100vh;
    display: flex;
    align-items: center;
    justify-content: center;
    background: linear-gradient(160deg, var(--cbt-primary) 0%, var(--cbt-primary-dark) 100%);
}
.cbt-auth-card {
    background: var(--cbt-surface);
    border-radius: 16px;
    max-width: 420px;
    width: 100%;
    padding: 2.25rem 2rem;
    box-shadow: 0 20px 50px rgba(15,23,41,.25);
}

/* ---------- Exam-taking mode ---------- */
.cbt-exam-shell { background: var(--cbt-bg); min-height: 100vh; }
.cbt-question-nav-btn {
    width: 38px; height: 38px;
    border-radius: 8px;
    border: 1px solid var(--cbt-border);
    background: var(--cbt-surface);
    font-size: .8rem;
    font-weight: 600;
}
.cbt-question-nav-btn.answered { background: var(--cbt-success); color: #fff; border-color: var(--cbt-success); }
.cbt-question-nav-btn.flagged { background: var(--cbt-warning); color: #fff; border-color: var(--cbt-warning); }
.cbt-question-nav-btn.current { outline: 2px solid var(--cbt-primary); outline-offset: 2px; }

.cbt-timer {
    font-variant-numeric: tabular-nums;
    font-weight: 700;
}