You can change the thickness of staff lines project-wide.
Procedure
- Press Ctrl/Cmd-Shift-E to open Engraving Options.
- In the category list, click Staves.
- In the Staff Lines section, change the value for Staff line thickness.
- Click Apply, then Close.
Result
The thickness of staff lines project-wide is changed. Increasing the value for Staff line thickness makes staff lines thicker, decreasing the value makes staff lines thinner.