Where is color settings for this part of window's border in i3's config?

[arash28134]

When I open a new window in a workspace with no other windows, there's a blue strip on window's border that is lighter than rest of the border:

(it goes away when I open another window in the workspace and the whole border becomes single-colored)

I read through the documentation but still couldn't find what part of i3's config file changes the color of this strip.

[okraits]

https://i3wm.org/docs/userguide.html#client_colors

The right border in this case is painted in the indicator color.