If you thought about that

it's not a line, it's a box-shadow from header. I wish you would learn one day how to use Code Inspector from your browser to get know answer faster, it's my a big wish. And that grey "shadow line" is from your template "Astroid Zero_altius" styles, not SPPB.
In case of ... https://www.joomshaper.com/blog/which-line-of-css-i-have-to-edit-code-inspector-in-action || useful for CSS investigations.
If you want to hide (reduce) white space between header and slider image, in first Row change padding value from 10px to 0px. And will be much better after. Is it all clear now?
