When adding the second solution to this answer, I initially tried to compile with XeLaTeX so that I could use fontspec. However, that didn't work, although compiling with LuaLaTeX did.
I've now managed to create an MnWE: there is a problem, it seems with my use of inner color and outer color with the XeTeX engine. The following example gives the expected output when compiled with pdfLaTeX or LuaLaTeX, but not XeLaTeX.
\documentclass[tikz, border=10pt, multi]{standalone}
\begin{document}
\begin{tikzpicture}
\node [inner color=yellow, outer color=red] {Node};
\end{tikzpicture}
\end{document}
Output with LuaLaTeX and pdfLaTeX:

Output with XeLaTeX:

Is it possible to use this kind of fill with XeLaTeX? If so, how? If not, is any engine-agnostic alternative available?