\node[font=\bfseries, circle, fill=debianblue!50, above=3em
of null.north, xshift=2em, text=darkgray] (H) {H};
\draw[arrow, draw=darkgray] (null) -- (H);
\node[font=\bfseries, circle, fill=debianblue!50, above=3em
of null.north, xshift=2em, text=darkgray] (H) {H};
\draw[arrow, draw=darkgray] (null) -- (H);