If you want to use compare pages on your store you have to add some CSS code to the style that is associated with the Category List page and the Product Details page.
Note: Styles are associated to pages using themes. Most of the time the default style will be the style that has been associated to the Category List and Product Detail pages but this might not always be true. You can see which style is used by those pages by editing the default theme for your store and checking the Apply Styles tab. See Using Themes to Apply Styles to Pages for more information.
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