CSS: Heading underline, width control.

[Richard238]

I’m looking at the border-bottom in the ‘Lower third’ region.
Currently, the line below the heading, when enabled, crosses the entire display.

The CSS attributeborder-bottom-width changes not the width, but the height of the line, not what I want.

How then, do I get border-bottom in lower-third, to be just the width of what it’s showing, and not the entire width of the display?

Thank you.