* {
  box-sizing: border-box;
  margin: 0;
  padding: 0;
}

body {
  font-family: -apple-system, BlinkMacSystemFont, "Segoe UI", Roboto, sans-serif;
  background-color: #f4f6f8;
  display: flex;
  justify-content: center;
  align-items: center;
  min-height: 100vh;
  padding: 15px;
}

.card {
  background: #ffffff;
  padding: 24px;
  border-radius: 12px;
  box-shadow: 0 4px 12px rgba(0, 0, 0, 0.08);
  width: 100%;
  max-width: 500px;
}

.hidden {
  display: none !important;
}

/* 登录框样式 */
.login-box {
  display: flex;
  gap: 10px;
  margin-top: 15px;
}

.login-box input {
  flex: 1;
  padding: 10px;
  border: 1px solid #cbd5e0;
  border-radius: 6px;
  font-size: 15px;
}

.error-msg {
  color: #e53e3e;
  font-size: 14px;
  margin-top: 8px;
}

/* 顶部导航 */
.header {
  display: flex;
  justify-content: space-between;
  align-items: center;
  margin-bottom: 20px;
  padding-bottom: 12px;
  border-bottom: 1px solid #edf2f7;
}

/* 按钮通用样式 */
.btn {
  padding: 8px 16px;
  border: none;
  border-radius: 6px;
  cursor: pointer;
  font-size: 14px;
  font-weight: 500;
  transition: opacity 0.2s;
}

.btn:hover {
  opacity: 0.9;
}

.btn-primary { background-color: #3182ce; color: white; }
.btn-secondary { background-color: #edf2f7; color: #4a5568; }
.btn-outline { background-color: transparent; border: 1px solid #cbd5e0; color: #4a5568; }
.btn:disabled { opacity: 0.4; cursor: not-allowed; }

/* 选项样式 */
.options-group {
  display: flex;
  flex-direction: column;
  gap: 10px;
  margin: 20px 0;
}

.option-btn {
  width: 100%;
  text-align: left;
  padding: 12px;
  border: 1px solid #e2e8f0;
  border-radius: 8px;
  background: white;
  font-size: 15px;
  cursor: pointer;
}

.option-btn.correct { background-color: #c6f6d5; border-color: #38a169; color: #22543d; }
.option-btn.wrong { background-color: #fed7d7; border-color: #e53e3e; color: #742a2a; }

/* 提示框 */
.feedback-box {
  padding: 10px 14px;
  border-radius: 6px;
  font-size: 14px;
  margin-bottom: 15px;
  text-align: center;
  font-weight: 500;
}

.feedback-box.correct { background-color: #f0fff4; color: #276749; border: 1px solid #9ae6b4; }
.feedback-box.wrong { background-color: #fff5f5; color: #9b2c2c; border: 1px solid #feb2b2; }

.nav-buttons {
  display: flex;
  justify-content: space-between;
  margin-top: 15px;
}