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There are some strings defined for part, chapter, and sections. Each of these strings start with the " | " character as a separator. For example, in the header of a page, you may find a sequence of strings:
My Publication | Introduction | Getting Started 
  1. "My Publication" is the value of the maptitle string.
  2. "Introduction" is the value of the chaptertitle string.
  3. "Getting Started" is the value of the sectiontitle string.

There might be cases where you want to change this separator. You will need to recompose the header content using the above string sets. Suppose you want to use " - " as a separator. In your customization CSS, add the following CSS rule:

*[class ~= "topic/topic"][is-part] > *[class ~= "topic/title"] {
    string-set: parttitle " - " counter(part, upper-roman) " - " content(), chaptertitle  ""; /* Avoid propagating a past chapter title on a new part */
}
*[class ~= "topic/topic"][is-chapter]:not([is-part]) > *[class ~= "topic/title"] {
    string-set: chaptertitle " - " counter(chapter) " - " content();
}

If you enabled the deep numbering for chapters and subsections, then use:

/* 
 * Alter the string sets that are shown in the header of the page.
 */
*[class ~= "map/map"][p|numbering^='deep'] *[class ~= "topic/topic"][is-part] > *[class ~= "topic/title"] {
    string-set: parttitle " - " counter(part, upper-roman) " - " content(), chaptertitle  "" ; /* Avoid propagating a past chapter title on a new part */
}
*[class ~= "map/map"][p|numbering^='deep'] *[class ~= "topic/topic"][is-chapter]:not([is-part]) > *[class ~= "topic/title"] {
    string-set: chaptertitle " - " counter(chapter) " - " content(), sectiontitle "" ;
}
*[class ~= "map/map"][p|numbering^='deep'] *[class ~= "topic/topic"][is-chapter]:not([is-part]) > *[class ~= "topic/topic"] > *[class ~= "topic/title"] {
    string-set: sectiontitle " - " counter(chapter) "." counter(section1) " - " content();
}