How to get panel spacers that are visible as a thin line?

Two ideas:
1) Try modifying the spacer widget (check out the code and fiddle with its qml) to include the line
2) Try modifying the theme's handling of spacer widgets (copy the theme to the local directory for themes and fiddle with its svgs) to display the line

Too technical for a non-programmer like me.

I thought there would be easier solutions like an option in the spacer widget or a separate 'divider' widget.