Which style config item powers the colour of the sub-tab line?

[PANstudio]

When I open a link in a tab, it spawns underneath the parent tab as a sort of sub-tab. In Sidebery, this is shown as follows (note the very faint line to the left of the child tab):

I can't discern from the style editor which config property controls the colour of that line, as I'd like to change it. Is this possible?

[piousdeer]

--tabs-normal-fg