Theme element name for border lines

Well I don’t know why I can’t seem to put this one to bed and just give up…  LOLOL!
Does anyone know the theme element name for the border lines, shown in white here?
I promise that if I can get this theme done correctly, I will make the theme publicly available but also I will write up a document about all the craziness I am learning about themes - so hopefully the next guy might be spared some of the indescribably frustration I’ve been experiencing! LOL

Beuller? Beuller?

PLEEEEEZE somebody helllp meeee!!!