Avec `text=black` :
\documentclass[tikz, border = 10pt]{standalone}
\usetikzlibrary{shapes.geometric, backgrounds,
positioning-plus, node-families, calc}
\tikzset{
basic box/.style = {
shape = rectangle,
align = center,
draw = #1,
fill = #1!25,
rounded corners,
text=black},
header node/.style = {
Minimum Width = header nodes,
font = \strut\Large\ttfamily,
text depth = +0pt,
fill = white,
draw,
text=black},
header/.style = {%
inner ysep = +1.5em,
append after command = {
\pgfextra{\let\TikZlastnode\tikzlastnode}
node [header node] (header-\TikZlastnode) at (\TikZlastnode.north) {#1}
node [span = (\TikZlastnode)(header-\TikZlastnode)]
at (fit bounding box) (h-\TikZlastnode) {}
},
},
hv/.style = {to path = {-|(\tikztotarget)\tikztonodes}},
vh/.style = {to path = {|-(\tikztotarget)\tikztonodes}},
fat blue line/.style = {ultra thick, blue}
}
\begin{document}
\begin{tikzpicture}[node distance = 1.2cm, thick, nodes = {align = center},
>=latex]
\node[Minimum Width = loop, shape = rectangle, rounded corners, fill = red,text=black] (pic)
{Pictures} ;
\node[Minimum Width = loop, shape = rectangle, rounded corners, fill = yellow, below = of pic,text=black] (data)
{Data Center} ;
\begin{scope}[on background layer]
\node[fit = (pic)(data), basic box = blue,
header = ahem] (test) {ahem};
\end{scope}
\node[left = of test, basic box = green, header = header-text-2]
(cs) {Text and text and text and text\\
and text\\and text again};
\end{tikzpicture}
\end{document}\end{document}
![alt text][1]
[1]: https://texnique.fr/osqa/upfiles/document_1m1WqBl.png
