Introducing KB New Posts, a plugin inspired by this support thread 
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What it does
When somebody visits your blog’s main page, the plugin sets a cookie recording the time of their visit. If they come back a few days later, and you’ve written a new post (or many new posts) since their last visit, the plugin adds a CSS class to the new posts so that you can highlight them, or add a background image, or change the font size, or whatever you want to do to new posts. (Want to read the rest of this post? Click here.)
