message: style(menu): add background color and z-index vars
diff: docs-menu {
  + background-color: var(--background-color);
  border-right: solid 1px var(--line-rule-color);
  display: flex;
  flex-direction: column;
  @@ -7,6 +8,7 @@ docs-menu {
  position: fixed;
  top: 0;
  width: var(--menu-width);
  + z-index: var(--menu-z-index, initial);
  }
  docs-menu > header {
  