Boldface arrows in tikzcd

Question

I want to make some of the arrows in my tikzcd diagram bold, but not all of them. How? Say, just the horizontal arrows in the following MWE:

\documentclass{article}
\usepackage[utf8]{inputenc}
\usepackage{tikz-cd}

\begin{document}



\[\begin{tikzcd}[column sep=small, row sep=small]
\ddots \arrow[rd] &                                             &                      &                                             & M_2 \arrow[rd, hook] &                                             &                      &                              &             &   \\
\cdots \arrow[r]  & F_3 \arrow[rr, "d_3"] \arrow[rd, two heads] &                      & F_2 \arrow[rr, "d_2"] \arrow[ru, two heads] &                      & F_1 \arrow[rd, two heads] \arrow[rr, "d_1"] &                      & F_0 \arrow[r, "\varepsilon"] & M \arrow[r] & 0 \\
                  &                                             & M_3 \arrow[ru, hook] &                                             &                      &                                             & M_1 \arrow[ru, hook] &                              &             &  
\end{tikzcd}\]

\end{document}

Answer 1

tikz-cd is based on the package tikz. theas enable to define styles of tikzcd images elements similarly as at pure tikz pictures. specific how to change forms of tikzcd images see tikz-cd documentation.

\documentclass[varwidth,margin=3mm]{standalone}
\usepackage{tikz-cd}
\usetikzlibrary{arrows.meta}

\begin{document}
\[
\tikzset{LA/.style = {draw=red, % just to demonstrate, where LA is used
                      line width=#1, -{Straight Barb[length=3pt]}},
         LA/.default=1pt
        }
\begin{tikzcd}[column sep=small, row sep=small]
\ddots \arrow[rd] &                                             &                      &                                             & M_2 \arrow[rd, hook] &                                             &                      &                              &             &   \\
\cdots \arrow[r,LA]  & F_3 \arrow[rr,LA=0.5pt, "d_3"] \arrow[rd, two heads] &                      & F_2 \arrow[rr, "d_2"] \arrow[ru, two heads] &                      & F_1 \arrow[rd, two heads] \arrow[rr, "d_1"] &                      & F_0 \arrow[r, "\varepsilon"] & M \arrow[r] & 0 \\
                  &                                             & M_3 \arrow[ru, hook] &                                             &                      &                                             & M_1 \arrow[ru, hook] &                              &             &
\end{tikzcd}
\]
\end{document}