Operator font in beamer/metropolis

Question

When using the metropolis theme for beamer, I get text and math in Fira Sans, except for math operators. I compile the following MWE using LuaTeX.

\documentclass{beamer}

\usetheme{metropolis}

\begin{document}

  \begin{frame}
    Ordinary text.
    \[ a^2 + b^2 = \sin^2(x) - \mathsf{y} + \mathrm{z}  \]
  \end{frame}

\end{document}

The outcome for me is that a, b, 2 and x are set in Fira Sans Light Italic, and y is set in Fira Sans Light, but sin is set in the sans font of (I guess) Latin Modern. (The z is also in Latin Modern, but this is expected behaviour from beamer.)

Unsurprisingly, I get the same result if I replace \usetheme{metropolis} with

\usepackage[no-math]{fontspec}
\setsansfont[ItalicFont={Fira Sans Light Italic},
  BoldFont={Fira Sans},
  BoldItalicFont={Fira Sans Italic}]%
  {Fira Sans Light}

I guess that this is to do with beamer's patching parts of the math fonts to match the text font (see this previous question). However, I don't understand the font system well enough to modify this.

There was a similar question asked before, but I've no idea how to apply that answer when using fontspec, or if this is even possible.

Is there a way to change the operator font to match the upright/sans math font (i.e., Fira Sans Light)?

Answer 1

Maybe changing the operator font could be a workaround:

% !TeX TS-program = xelatex
\documentclass{beamer}
\usetheme{metropolis}
\DeclareSymbolFont{sfoperators}{OT1}{FiraSans-LF}{l}{n}
\makeatletter
    \renewcommand{\operator@font}{\mathgroup\symsfoperators}
\makeatother
\begin{document}
\begin{frame}
    Ordinary text.
    [ a^2 + b^2 = \sin^2(x) - \mathsf{y} + \mathrm{z}  ]
  \end{frame}
\end{document}

Alternatively, you could use the moloch theme, a more modern fork of the metropolis theme. moloch does not change the fonts, so you will get a consistent set by default (computer modern sans):

\documentclass{beamer}
\usetheme{moloch}% modern fork of the metropolis theme
\begin{document}
\begin{frame}
    Ordinary text.
    [ a^2 + b^2 = \sin^2(x) - \mathsf{y} + \mathrm{z}  ]
  \end{frame}
\end{document}