On the normal horizontal tabs, there is a horizontal coloured line across the top of the tab and across the bottom of the tab, one to indicate the active tab, and one to indicate the container.

With MrOtherGuy’s vertical tabs, how can one make those coloured lines/bars be vertical, one on one side of the tab, and one on the other side of the tab?

To get an idea of what this is, see the screenshots for this extension: https://addons.mozilla.org/en-US/firefox/addon/tabcenter-reborn/ .