Is this default meant to be in this document or in all future documents?
ALL DOCUMENTS
Do you intend to change existing lines or just new ones?
NEW ONES
Are you using shapes from stencils to drag lines onto the page or just using the new line from the Home>Tools dropdown button?
NEW LINE FROM THE HOME…
Are you controlling the look in the docs using themes?
NO

I followed your instructions and was able to change the default for the current doc.
If you have the time could you share the technique to set the default for all docs.
Thank you.