Changing default font weight for FiraSans and smath

Question

I'd like to use FiraSans as default for my documents. The default values work fine, but I'd like FiraSans Medium as my default bold font and FiraSans Light as my default regular font.

I already tried \renewcommand{\seriesdefault}{l} and \renewcommand{\bfdefault}{mb} but it has no effect (this worked while I used OpenSans - but with FiraSans only the regular font is affected, but the bold font stays untouched).

The several font series can be looked up here: ftp://ftp.rrzn.uni-hannover.de/pub/mirror/tex-archive/fonts/fira/latex/FiraSans.sty

Further, I have a problem with sfmath as it uses FiraSans Regular even when I use \renewcommand{\seriesdefault}{l} for FiraSans Light. I'd like to have the math font in light weight, too. Is that possible?

MWE:

% ATTRIBUTES
\documentclass{scrbook}
\usepackage[ngerman]{babel}
\usepackage[utf8]{inputenc}
\parskip\baselineskip
\parindent0pt

% FONT
\usepackage[T1]{fontenc}
\usepackage[sfdefault]{FiraSans}
\usepackage{sfmath}

% MATH
\usepackage{mathtools}

\begin{document}

\subsubsection{How it is now (default)}

\textbf{This is bold} and this is regular font weight with $\sin(123456789)$ math.

\subsubsection{Hot I'd like it}

{\firalight But I'd like Fira Light as my standard regular font.}

{\firamedium And Fira Medium as my standard bold font.}

{\firalight And \texttt{sfmath} shall adapt Fira Light for $\sin(123456789)$ math, because at the moment it uses Fira Regular for math.}

\end{document}

Answer 1

Something like this? FiraSans has a rather non-standard font setup, and sfmath hard-codes the font series. You may need to fish out and redo some more declarations but this should give you the idea:

\documentclass{scrbook}
\usepackage[utf8]{inputenc}
\usepackage[T1]{fontenc}
\usepackage[sfdefault,light]{FiraSans}
\usepackage{sfmath}
\usepackage{mathtools}
\makeatletter
\def\bfseries@sf{m}
\SetSymbolFont{operators}{normal}{\math@encoding}{\math@sfdefault}{l}{n}
\SetSymbolFont{operators}{bold}{\math@encoding}{\math@sfdefault}{m}{n}
\DeclareSymbolFont{SFMath}{\math@encoding}{\math@sfdefault}{l}{sl}
\SetSymbolFont{SFMath}{normal}{\math@encoding}{\math@sfdefault}{l}{sl}
\SetSymbolFont{SFMath}{bold}{\math@encoding}{\math@sfdefault}{m}{\mathnormal@bold@shape}
\makeatother
\begin{document}

\subsubsection{How it is now (default)}

\textbf{This is bold} and this is regular font weight with $\sin(123456789)$ math.

\end{document}

Note that it is debatable whether you should typeset a document entirely in sans or maths in it at all.