To edit an existing HTML style, simply double-click its name in the HTML Styles panel (disable the Auto Apply option to prevent the style you're editing from being applied to the current selection). You cannot edit the styles that come built into the HTML Styles panel (i.e., Clear Selection Style and Clear Paragraph Style).
Once inside the Define HTML Styles dialog box, make any desired changes to the style definition. If you modify an HTML style, the changes are not propagated to text to which the style was previously applied. You must reapply the style manually to any text you wish to update.
Styles are stored in the styles.xml file inside the Library folder within your site's root folder. The file's contents look something like this:
<mm:style name="Add Bold" type="char" apply="add" bold /> <mm:style name="green" type="char" apply="replace" color="#00CC99" bold italic />
The styles.xml file can be edited in any text editor and even copied to another site's Library folder if you want to copy your favorite styles to a new site.