There are a couple of issues with your latest CSS example:
1: Your example CSS rule has a # character before the body name in its CSS Selector, and as shown in the documentation if just linked to, that character has special meaning when added to the start of a selector.
It instructs the web-browser that that rule targets any HTML element that has been assigned an ID attribute with a value of “body”…
<div id="body"> ...this element would be targeted... </div>
<p id="body"> ...as would this element...</p>
However, your previous CSS rule was targeting an element that had an element-type of body instead…
<body> ...the content of the HTML page's body element... </body>
Remove the # character from the start of the #body CSS selector should fix which HTML element is being targeted.
2: The back-slash \ characters you’re using to delimit the folder & file names in your URL aren’t escaped, and as I explained in your previous question about this same topic, you should be using forward-slash / characters as the delimiter.
eg. Change the following line in your CSS…
background-image: url("C:\TrueFolder\images\wallpaper.jpg");
…to be the following instead…
background-image: url("C:/TrueFolder/images/wallpaper.jpg");
…and the back-ground image on your local hard-drive should now be locatable while using the Twine 2.x application’s Test & Play options.
However, using a Absolute URL like your C:/TrueFolder/images/wallpaper.jpg will cause issues for anyone else viewing the Story HTML file you generate, because they aren’t likely to be storing that HTML file in a folder named TrueFolder on the C drive of their comuter.
I strongly suggest you switch to using Relative URLs when accessing your project’s media files…
background-image: url("images/wallpaper.jpg");
…even though that will mean you will no longer be able to see/hear the output of those files while using the Twine 2.x application’s Test & Play options, and that you will need to use the Publish to File option instead. Because doing this will allow others to access those media files when you release the Story HTML file you generate.