这个问题不是WordPress特有的，所以对于StackOverflow来说可能更好。我们可以通过建议使用wp_enqueue_script 将脚本添加到页面，但最终这是一个JavaScript问题。

我的回答是基于this article

// Get the local time of the visitor
var localTime = new Date();
var localHour = localTime.getHours();
            
// Bool to determine if it is night or day
var isNight = ( 18 <= localHour || 8 >= localHour ) ? true : false;
            
// Object containing URL links to images 
const  backgroundURLs = {
    day: \'https://placekitten.com/1960/1960\',
    night: \'https://placekitten.com/1961/1961\',
};
                
// Sets the background image
const setBackground = (image) => {
  // wait for window to be loaded to ensure body is present, 
  // if you are checking this elsewhere you can remove the window.onload check
  window.onload = function() {
     document.body.style.background = "url(\'"+backgroundURLs[image]+"\')";
  }
};
            
// Actually set the background based on the bool and URL object. 
if ( isNight ) {
  setBackground(\'night\');
} else {
  setBackground(\'day\');
}