Light and Dark Notebooks

DiagramPlot and DrawDiagram draw with black, white and greys only, and follow the appearance of the notebook:

• Mathematica 14.3 or later: lines and text are dark on a light notebook and light on a dark one (through LightDarkSwitched); what hides the lines under it (the inside of a blob, of an external point, the clearing behind the label of a hatched or dotted blob) takes the colour of the background, and the fill "Grey" of a blob is darker on a dark notebook.
• Before 14.3: the pictures have the colours of a light notebook, also on a dark one.

The option "Appearance" forces one look, on a background of its own (white, or the dark grey of a dark notebook):

Needs["FunctionalDiagrams`"]
Options[DiagramPlot, "Appearance"]

{Appearance -> Automatic}

"Appearance" -> "Light" or "Dark" is meant for a picture exported for a page of the other kind, for example a picture for slides with a dark background:

DeclareSource[J]; DeclareField[phi, J]; DeclareSymmetric[D0]; DeclareCoupling[lambda];
Zb = Exp[1/2 IntegralOver[J[x] D0[x, y] J[y], {x, y}]];
phi4 = lambda/24 IntegralOver[phi[z]^4, {z}];
propagator = PerturbativeCorrelationFunction[Zb, {phi[a], phi[b]}, phi4, 1];
Export["propagator_dark.png", DiagramPlot[propagator, {a, b}, "Appearance" -> "Dark"]]

propagator_dark.png

For a whole session, SetOptions sets the default of both functions:

SetOptions[{DiagramPlot, DrawDiagram}, "Appearance" -> "Dark"];

TikZ and the web

DiagramToTikZ ignores "Appearance": its code is for white pages, black lines on white. A picture made from it for a page that may be light or dark, a README or a wiki, reads on both when it is placed on a white card: the pictures of the README and of this wiki are SVG files with a white rounded rectangle under the drawing (Development/make_readme_figure.sh and Development/check_wiki.wl show how). See Pictures in LaTeX.