The Styles pane shows which CSS style rules apply to the currently selected element. A link next to a style rule will open the file where that particular style is defined. This can be either a CSS file or the source file of a section if local formatting was used (see Styling and formatting).
A crossed-out style rule signals that it was overruled by another style rule. This happens when:
A more specific, and therefore more important rule, is encountered for the same element. See Using a more specific CSS rule to learn more about the specificity of style rules.
A rule with the same importance is read after the first rule. Not only is the order of the rules in a CSS file important, but also the order in which the style sheets are read. The style sheets that are included with a section are read in the specified order; see Applying a style sheet to a section.