Currently the focus rect for a tab is drawn on all sides, perhaps it would look nicer if it was not drawn on the bottom?
Gtk tabs:

Current Kvantum style of Communitheme tabs:

+1 from me. @Feichtmeier is this within your CSS skills?
@Feichtmeier It's possible to change the focus style individually for each element.
It's also possible to reproduce the Kvantum style, but it'll be a very hacky way. :wink:

You can try it by pasting the code into the GTK+ Inspector:
notebook:focus header.top tab:checked {
outline: none;
border-color: transparent transparent @theme_base_color;
background-image: image(@theme_base_color), linear-gradient(to bottom, @theme_selected_bg_color, @borders);
background-origin: padding-box, border-box;
background-clip: padding-box, border-box;
}
I鈥檓 not sure if it's really consistent with other focus rings.
I think there are other options too like this:

Most helpful comment
I鈥檓 not sure if it's really consistent with other focus rings.
I think there are other options too like this: