White Lines between lesson header and block

I ran into this too and it turned out to be a tiny gap caused by the lesson header container and the first block not “touching” cleanly when dark backgrounds are used. In most cases it’s either a default background color showing through, or a 1px margin/padding or border between the header and the first block, which becomes more noticeable when you change header height.

What fixed it for me was setting the lesson background to match the header color and removing spacing at the top of the first block. If your platform allows custom CSS, try forcing the header and first block to have no bottom or top margin and no border, and set the background behind them to the same dark color. Also check if there is a “separator” or “block padding” setting enabled in the theme, because that can create the thin white line only at the header boundary.