De-emphasise dividing lines when rendering German Lute Tablature

[musicog]

This issue concerns the rendering of German Lute Tablature (GLT) as implemented by @paul-bayleaf and currently available in branch develop-tab.

The image above displays the current rendering produced for an example GLT encoding as of PR #3581. It's wonderful to see this rendering in Verovio! However, having run the output by @Kateryna333 (Kateryna Schöning, P.I., E-LAUTE), I would like to forward her suggestion to de-emphasise the dividing lines between systems, by:

1. slightly lowering the lines to ensure no contact between the note glyphs and the dividers, and
2. making the dividing lines lighter in colour so as not to dominate the image

This would allow the lines to better serve in their intended role as reading aids, without becoming distracting.

It's possible that 1. is font-dependent -- the glyphs are sufficiently separated from the lines when the Bravura font is specified: