The following variables can control how working hours are displayed in the timezone planner.
casual-timezone-working-hours-range will set the range (start, stop) of working hours. The values are integers that map to 24-hour time (0..23).
casual-timezone-working-hour-glyph will set the glyph used to denote a working hour (default is ☼).
casual-timezone-planner-working-highlight will set the face used to highlight a working hour.
These variables can be customized via the Transient menu casual-timezone-settings-tmenu.