currentThemeLink变量用于跟踪当前应用的主题的CSS文件。在添加新主题的CSS文件之前，代码会检查之前是否加载了主题。如果是，它将从文档中删除前一个链接标记。这可以确保一次只应用一个主题的CSS文件，并且在切换主题时正确处理清理。
请记住将/path-to-themes/替换为主题CSS文件的实际路径。

// Variable to store the reference to the currently loaded theme CSS file
let currentThemeLink = null;

function loadThemeCSS(themeName) {
  // Create a new link element for the selected theme
  const newThemeLink = document.createElement('link');
  newThemeLink.rel = 'stylesheet';
  newThemeLink.type = 'text/css';
  newThemeLink.href = `/path-to-themes/${themeName}.css`; // Replace with the actual path

  // If a theme was previously loaded, remove it before adding the new one
  if (currentThemeLink) {
    document.head.removeChild(currentThemeLink);
  }

  // Add the new theme's CSS file to the document
  document.head.appendChild(newThemeLink);

  // Update the reference to the current theme link
  currentThemeLink = newThemeLink;
}

// Example usage when the user selects a theme:
function handleThemeChange(themeName) {
  loadThemeCSS(themeName);
}

// Render your UI element for theme selection
<button onClick={() => handleThemeChange('theme-light')}>Light Theme</button>
<button onClick={() => handleThemeChange('theme-dark')}>Dark Theme</button>