Conditional selection of tikz style classes

I try to achieve that different style classes are chosen in a self-defined tikz function depending on the input number.

However, the \ifnum command doesn't seem to work as I expect it. The error message that I get is:

> thesis/image/outline_MWE.tex:46: Missing = inserted for \ifnum.
<to be read again> 
                   }
l.46    \makeoutlinefig{1}
                        
thesis/image/outline_MWE.tex:46: Missing number, treated as zero.
<to be read again> 
                   }
l.46    \makeoutlinefig{1}

MWE:

\documentclass[varwidth=true]{standalone}

    % \input{../preamble.tex}
    % \input{../colors.tex}

    \usepackage{tikz}
    \usetikzlibrary{shapes,arrows,arrows.meta, calc, decorations.markings, backgrounds,fit,positioning,plotmarks, intersections, patterns, intersections,decorations.text,external,decorations.pathreplacing}

\begin{document}

\DeclareRobustCommand{\makeoutlinefig}[1]{
        \centering
        \def\threebw{16.7cm}
        \begin{tikzpicture}     
            [auto, box/.style ={rectangle, 
                                % font= \tiny,
                                draw=black, 
                                thick, 
                                % fill=blue!30, 
                                text width=3.0cm,
                                minimum width=1.5cm,
                                align=center, 
                                rounded corners, 
                                minimum height=2.0cm, 
                                dashed, thick},
                    activebox/.style = {box, draw=red, 
                    thick,solid},       
                    node distance = 0.5cm,
                ]

            \node[style=\ifnum#1=1 activebox\else box\fi,
                        text width=\threebw] (b1) at (0,0) {\textbf{1. Chapter}};
            \node[style=\ifnum#1=2 activebox\else box\fi, 
                        text width=\threebw, below = of b1] (b2) {\textbf{2. Chapter}} ;

            \draw [-Stealth,ultra thick] (b1) -- (b2);
        \end{tikzpicture}
        }

    \makeoutlinefig{1}

    \end{document}

You can avoid the problem like this:

\documentclass[varwidth=true]{standalone}

\usepackage{tikz}
\usetikzlibrary{shapes,arrows,arrows.meta, calc, decorations.markings, backgrounds,fit,positioning,plotmarks, intersections, patterns, intersections,decorations.text,external,decorations.pathreplacing}

\begin{document}

\DeclareRobustCommand{\makeoutlinefig}[1]{

  \centering
  \def\threebw{16.7cm}
  
  \begin{tikzpicture}[
    auto, 
    box/.style = {
      rectangle, 
      draw=black, 
      thick, 
      text width=3.0cm,
      minimum width=1.5cm,
      align=center, 
      rounded corners, 
      minimum height=2.0cm, 
      dashed, 
      thick
    },
    activebox/.style = {
      box, 
      draw=red, 
      thick,
      solid
    },       
    node distance = 0.5cm,
  ]
  
    \ifnum#1=1
      \def\mystyle{activebox}
    \else
      \def\mystyle{box}
    \fi
    
    \node[\mystyle, text width=\threebw] (b1) at (0,0) {\textbf{1. Chapter}};
    
    \ifnum#1=2
      \def\mystyle{activebox}
    \else
      \def\mystyle{box}
    \fi            
    
    \node[\mystyle, text width=\threebw, below = of b1] (b2) {\textbf{2. Chapter}} ;
    
    \draw [-Stealth,ultra thick] (b1) -- (b2);
  \end{tikzpicture}
  
}

\makeoutlinefig{2}

\end{document}