/* ===== Reset ===== */
*, *::before, *::after {
  box-sizing: border-box;
  margin: 0;
  padding: 0;
}

html {
  -webkit-text-size-adjust: 100%;
  scroll-behavior: smooth;
}

img, svg {
  display: block;
  max-width: 100%;
}

button, input {
  font: inherit;
  color: inherit;
}

/* ===== Body ===== */
body {
  font-family: -apple-system, BlinkMacSystemFont, "Segoe UI", Roboto, Oxygen,
    Ubuntu, Cantarell, "Helvetica Neue", Arial, sans-serif;
  font-size: 1rem;
  line-height: 1.5;
  background-color: var(--color-bg);
  color: var(--color-text);
  transition: background-color var(--transition-normal),
              color var(--transition-normal);
  min-height: 100vh;
}

/* ===== Container ===== */
.container {
  max-width: 720px;
  margin-inline: auto;
  padding-inline: var(--space-md);
}

/* ===== Typography ===== */
h1, h2, h3, h4 {
  line-height: 1.25;
  font-weight: 600;
}

h1 { font-size: 1.75rem; }
h2 { font-size: 1.4rem; }
h3 { font-size: 1.15rem; }

p {
  margin-block: var(--space-sm);
  color: var(--color-text-secondary);
}

p:first-child { margin-block-start: 0; }
p:last-child  { margin-block-end: 0; }

/* ===== Screen Switching ===== */
.screen {
  display: none;
}

.screen.active {
  display: block;
}

/* ===== Header ===== */
.app-header {
  position: sticky;
  top: 0;
  z-index: 100;
  background-color: var(--color-surface);
  border-bottom: 1px solid var(--color-border);
  padding-block: var(--space-sm);
  transition: background-color var(--transition-normal),
              border-color var(--transition-normal);
}

.header-inner {
  display: flex;
  align-items: center;
  justify-content: space-between;
}

.app-title {
  font-size: 1.25rem;
  color: var(--color-primary);
}

/* ===== Utility ===== */
.visually-hidden {
  position: absolute;
  width: 1px;
  height: 1px;
  overflow: hidden;
  clip: rect(0, 0, 0, 0);
  white-space: nowrap;
}