/* basic reset */
:root { font-family: "Inter", system-ui, sans-serif; line-height: 1.5; }
body  { margin: 0 4vw 4vw; background: #fafbff; color: #222; }

.headline       { font-size: 1.75rem; margin: 1.25rem 0; }
.section-title  { margin: .75rem 0 .25rem; }

.canvas-wrap    { display: flex; gap: 2.5rem; flex-wrap: wrap; }
.card           { background: #fff; padding: 1rem; border-radius: 1rem;
                  box-shadow: 0 1px 4px rgba(0,0,0,.06); }
canvas          { border-radius: .5rem; image-rendering: pixelated;
                  border: 1px solid #ccd; }

.kernel-grid    { display: grid; grid-template-columns: repeat(3, 2.8em);
                  gap: .25em; margin-bottom: .6em; }
.kernel-cell    { display: flex; align-items: center; justify-content: center;
                  width: 2.8em; height: 2.8em; font-weight: 600; font-size: .9em;
                  color: #fff; border-radius: .3em; user-select: none; }

.sum-readout    { font-family: monospace; margin: .4em 0 1em; }

.controls       { display: flex; gap: .75rem; flex-wrap: wrap;
                  align-items: center; margin-top: .5rem; }
button, select  { padding: .35em .9em; border-radius: .4em; border: none;
                  background: #2563eb; color: #fff; font-weight: 600;
                  cursor: pointer; transition: background .15s; }
button:hover,
select:hover    { background: #1d4ed8; }
button:disabled { opacity: .55; cursor: default; }
.speed-label    { display: flex; align-items: center; gap: .4em; }
input[type=range]{ accent-color: #2563eb; }

#kernelCanvas   { display:block; margin-bottom: .8em; border: 1px solid #ccd;
                  border-radius: .25em; image-rendering: pixelated; }

                  .canvas-stack      { position: relative; width: 300px; height: 300px; }
.canvas-stack canvas {
  position: absolute; inset: 0;
  border-radius: .5rem;               /* keep rounded corners aligned */
}
#hilite           { pointer-events: none; }  /* overlay ignores clicks */

/* ─── theory section ───────────────────────────────── */
.theory            { margin-top: 3.5rem; }
.theory-headline   { font-size: 1.5rem; margin-bottom: 1.4rem; }

.theory-card       { background:#fff; padding:1.3rem 1.5rem; border-radius:1rem;
                     box-shadow:0 2px 6px rgba(0,0,0,.06); margin-bottom:1.4rem; }
.theory-card h4    { margin-top:0; font-size:1.1rem; }

.sym               { font-family: "Latin Modern Math", Cambria, serif; }

.theory .formula   { text-align: center; margin: .6rem 0; }

.theory figure     { margin: .8rem 0 0; text-align:center; }
.theory figcaption { font-size:.8rem; color:#666; margin-top:.4rem; }

@media (min-width: 850px){
  .theory-card     { display:grid; grid-template-columns: 1fr 1fr;
                     gap:1.2rem; align-items:center; }
  .theory-card h4  { grid-column: 1 / -1; }
}


/* ─── THEME VARIABLES ───────────────────────────────────── */
:root {
    --bg:       #f4f6fb;
    --fg:       #1e293b;
    --muted:    #64748b;
    --accent:   #4f46e5;
    --accent-2: #6366f1;
    --card-bg:  #ffffff;
    --radius:   1rem;
    --shadow:   0 2px 8px rgba(0,0,0,0.05);
    --transition: all 0.2s ease;
  }
  
  /* ─── GLOBAL RESET & LAYOUT ───────────────────────────── */
  body {
    margin: 0;
    padding: 0 1rem;
    background: var(--bg);
    color: var(--fg);
    font-family: "Inter", system-ui, sans-serif;
    line-height: 1.6;
  }
  .main-container {
    max-width: 960px;
    margin: 0 auto;
    padding: 2rem 0;
  }
  
  /* ─── HEADINGS & TEXT ─────────────────────────────────── */
  h2.headline,
  h3.docs-headline,
  h3.theory-headline {
    color: var(--accent);
    margin-bottom: 1rem;
  }
  .section-title {
    margin-top: 2rem;
    color: var(--accent-2);
    font-size: 1.2rem;
  }
  p, li {
    color: var(--muted);
  }
  a {
    color: var(--accent);
    text-decoration: none;
    transition: var(--transition);
  }
  a:hover {
    text-decoration: underline;
  }
  
  /* ─── CARD & CANVAS STYLING ───────────────────────────── */
  .card {
    background: var(--card-bg);
    border-radius: var(--radius);
    box-shadow: var(--shadow);
    padding: 1.5rem;
    transition: var(--transition);
  }
  .card:hover {
    transform: translateY(-4px);
    box-shadow: 0 4px 12px rgba(0,0,0,0.08);
  }
  
  .canvas-wrap {
    display: flex;
    gap: 2rem;
    flex-wrap: wrap;
    margin-bottom: 2rem;
  }
  .canvas-stack {
    position: relative;
    width: 300px; height: 300px;
  }
  /* only the original canvas has a white bg & inner shadow */
.canvas-stack #orig {
    position: absolute; inset: 0;
    border-radius: .5rem;
    background: #fff;
    box-shadow: inset 0 1px 3px rgba(0,0,0,0.05);
    image-rendering: pixelated;
  }
  
  /* the highlight overlay is fully transparent */
  .canvas-stack #hilite {
    position: absolute; inset: 0;
    border-radius: .5rem;
    background: transparent;
    pointer-events: none;
  }
  
  /* ─── KERNEL GRID ─────────────────────────────────────── */
  .kernel-grid {
    display: grid;
    grid-template-columns: repeat(3, 3rem);
    gap: .5rem;
    margin-bottom: 1rem;
  }
  .kernel-cell {
    display: flex;
    align-items: center;
    justify-content: center;
    background: var(--accent);
    color: #fff;
    border-radius: .5rem;
    font-weight: 600;
    cursor: default;
    transition: var(--transition);
  }
  .kernel-cell:hover {
    transform: scale(1.1);
    box-shadow: 0 1px 5px rgba(0,0,0,0.1);
  }
  
  /* ─── HEATMAP CANVAS ──────────────────────────────────── */
  #kernelCanvas {
    display: block;
    margin-bottom: 2rem;
    border-radius: .5rem;
    border: 1px solid #e2e8f0;
  }
  
  /* ─── SUM READOUT & CONTROLS ───────────────────────────── */
  .sum-readout {
    font-family: monospace;
    color: var(--fg);
    margin-bottom: 1.5rem;
  }
  .controls {
    display: flex;
    align-items: center;
    gap: 1rem;
    flex-wrap: wrap;
    margin-bottom: 3rem;
  }
  .controls button,
  .controls select {
    background: var(--accent);
    color: #fff;
    border: none;
    border-radius: .5rem;
    padding: .5rem 1rem;
    font-weight: 600;
    cursor: pointer;
    transition: var(--transition);
  }
  .controls button:disabled {
    opacity: 0.6;
    cursor: not-allowed;
  }
  .controls button:hover:not(:disabled),
  .controls select:hover {
    background: var(--accent-2);
  }
  .speed-label {
    display: flex;
    align-items: center;
    gap: .5rem;
  }
  input[type="range"] {
    accent-color: var(--accent);
  }
  
  /* ─── DOCS & THEORY SECTIONS ──────────────────────────── */
  .docs, .theory {
    background: var(--card-bg);
    padding: 2rem;
    border-radius: var(--radius);
    box-shadow: var(--shadow);
    margin-bottom: 2rem;
  }
  .docs-grid {
    display: grid;
    gap: 1.5rem;
    grid-template-columns: repeat(auto-fill, minmax(14rem,1fr));
  }
  .doc-card, .theory-card {
    background: #fafbff;
    padding: 1.2rem 1.5rem;
    border-radius: .75rem;
    transition: var(--transition);
  }
  .doc-card:hover, .theory-card:hover {
    background: #f0f4ff;
  }
  .doc-card header, .theory-card h4 {
    font-weight: 700;
    color: var(--accent);
    margin-bottom: .6rem;
  }
  .doc-step {
    background: var(--accent-2);
    color: #fff;
  }
  .theory .formula {
    background: #f3f4f6;
    padding: 1rem;
    border-radius: .5rem;
    overflow-x: auto;
    margin: 1rem 0;
  }
  
  /* ─── RESPONSIVE TWEAKS ───────────────────────────────── */
  @media (min-width: 800px) {
    .controls { justify-content: flex-start; }
    .theory-card {
      display: grid;
      grid-template-columns: 1fr 1fr;
      gap: 1.5rem;
      align-items: start;
    }
  }
  
  :root {
    /* blue-themed accents */
    --accent:   #1e3a8a;  /* dark blue */
    --accent-2: #3b82f6;  /* sky-blue */
    /* light-blue page background */
    --bg:       #eff6ff;
    /* card and text colors stay the same */
  }
  
  /* center & constrain everything */
  .main-container {
    max-width: 960px;
    margin: 0 auto;
    padding: 2rem 1rem;
  }
  
  /* adjust the overall page bg */
  body {
    background: var(--bg);
  }
  
  /* canvas-cards: center them within the container */
  .canvas-wrap {
    justify-content: center; /* was flex-start */
  }
  
  /* controls row: center on narrow screens, spaced on wide */
  @media (min-width: 600px) {
    .controls {
      justify-content: space-between;
    }
  }