When displaying surfaces in a cross section, the settings for how the surface line will appear are controlled by the symbology assigned to that surface. You can change the appearance in one of two ways.
1. By making edits to the Symbology assigned to each surface through Tools > Named Symbology Manager, edit the symbolgy and edit the Cross Section Line to reflect how you want the line to appear.
2. By changing the assigned symbology for the surface to a predefined symbology through Surface > Surface Properties > Advanced > Cross Sections - Symbology. Once the assigned symbology name is change and applied, when you create cross sections or update existing sections, the surface line will reflect the new symbology.