message: feat(md-enhance): add css variable for code demo header
diff: @use "@mr-hope/vuepress-shared/styles/arrow";
  @use "@mr-hope/vuepress-shared/styles/reset";
  +:root {
  + --code-demo-header-bg-color: #eee;
  +}
  +
  +#{hope-config.$dark-selector} {
  + --code-demo-header-bg-color: #333;
  +}
  +
  .code-demo-wrapper {
  margin: 10px 0;
  border: 1px solid var(--c-border, #eaecef);
  justify-content: space-between;
  padding: 8px 12px;
  + border-radius: 8px 8px 0 0;
  - background-color: var(--c-bg-light, #f3f4f5);
  + background-color: var(--code-demo-header-bg-color);
  text-align: left;
  // stylelint-disable selector-max-compound-selectors
  svg {
  position: relative;
  - top: 2px;
  - left: 2px;
  + top: 1px;
  + left: 1px;
  }
  // stylelint-enable selector-max-compound-selectors
  }
  