Error in tcolorbox Missing \endcsname inserted

Question

I am trying to a fancy theorem type for my notes. I like tcolorbox examples. But the one that I like most is giving me an error.

    Missing \endcsname inserted.

This one is on page 360 of the tcolorbox manual. I do not understand why I am getting this error and how to fix. There was a similar question

https://tex.stackexchange.com/questions/174017/compile-error-missing-endcsname-inserted-with-code-and-tcolorbox

But it did not solve my problem. The result should be like this

I have added the code below. Please help me.

    \documentclass[11pt]{article}
    \usepackage{varioref}
    \usepackage{cleveref}
    \tcbuselibrary{skins}

    \newtcbtheorem[use counter from=Definition]{YetAnotherTheorem}{Theorem}%
      {theorem style=change apart,enhanced,arc=0mm,outer arc=0mm,
      boxrule=0mm,toprule=1mm,bottomrule=1mm,left=1mm,right=1mm,
      titlerule=0mm,toptitle=0mm,bottomtitle=1mm,top=0mm,
      colframe=red!50!black,colback=red!5!white,coltitle=red!50!black,
      title style={top color=yellow!50!white,bottom color=red!5!white,
        middle color=yellow!50!white},
      fonttitle=\bfseries\sffamily\normalsize,fontupper=\normalsize\itshape,
      }{theo}


    \begin{document}
    \begin{YetAnotherTheorem}{Mittelwertsatz f\"{u}r $n$ Variable}{mittelwertsatz_n2}% Es sei $n\in\mathbb{N}$, $D\subseteq\mathbb{R}^n$ eine offene Menge und
    $f\in C^{1}(D,\mathbb{R})$. Dann gibt es auf jeder Strecke
    $[x_0,x]\subset D$ einen Punkt $\xi\in[x_0,x]$, so dass gilt
      \begin{equation*}
      f(x)-f(x_0) = \operatorname{grad} f(\xi)^{\top}(x-x_0)
      \end{equation*}
    \end{YetAnotherTheorem}

    \end{document}

Answer 1

This is too long for a comment. You have several issues:

• you are not loading tcolorbox package
• you need to add \tcbuselibrary{theorems}
• you have not defined \mathbb
• the use counter from=Definition does not work because you have not defined a definition environment. As per the comments, I have replaced this with no counter to remove the numbering.

The following code compiles and perhaps does what you want:

\documentclass[11pt]{article}
\usepackage{amssymb}
\usepackage{varioref}
\usepackage{tcolorbox}
\tcbuselibrary{skins,theorems}
\usepackage{cleveref}

\newtcbtheorem[no number]{YetAnotherTheorem}{Theorem}%
  {theorem style=change apart,enhanced,arc=0mm,outer arc=0mm,
  boxrule=0mm,toprule=1mm,bottomrule=1mm,left=1mm,right=1mm,
  titlerule=0mm,toptitle=0mm,bottomtitle=1mm,top=0mm,
  colframe=red!50!black,colback=red!5!white,coltitle=red!50!black,
  title style={top color=yellow!50!white,bottom color=red!5!white,
    middle color=yellow!50!white},
  fonttitle=\bfseries\sffamily\normalsize,fontupper=\normalsize\itshape,
  }{theo}

\begin{document}

  \begin{YetAnotherTheorem}{Mittelwertsatz f\"{u}r $n$ Variable}{mittelwertsatz_n2}% Es sei $n\in\mathbb{N}$, $D\subseteq\mathbb{R}^n$ eine offene Menge und
  $f\in C^{1}(D,\mathbb{R})$. Dann gibt es auf jeder Strecke
  $[x_0,x]\subset D$ einen Punkt $\xi\in[x_0,x]$, so dass gilt
    \begin{equation*}
    f(x)-f(x_0) = \operatorname{grad} f(\xi)^{\top}(x-x_0)
    \end{equation*}
  \end{YetAnotherTheorem}

\end{document}

The output is:

Edit To remove both the theorem name and number and just have the box itself, it is better to use newtcolorbox, to give:

Here is the updated code:

\documentclass[11pt]{article}
\usepackage{amsmath,amssymb}
\usepackage{varioref}
\usepackage{tcolorbox}
\tcbuselibrary{skins}
\usepackage{cleveref}

\newtcolorbox{YetAnotherTheorem}[1]%
  {enhanced,arc=0mm,outer arc=0mm,
  boxrule=0mm,toprule=1mm,bottomrule=1mm,left=1mm,right=1mm,
  titlerule=0mm,toptitle=0mm,bottomtitle=0mm,top=0mm,
  colframe=red!50!black,colback=red!5!white,coltitle=red!50!black,
  colbacktitle=yellow!50!white,colback=red!5!white,
  title=#1,
  fonttitle=\bfseries\sffamily\normalsize,fontupper=\normalsize\itshape,
  }

\begin{document}

\begin{YetAnotherTheorem}{Mittelwertsatz f\"{u}r $\mathbf{n}$ Variable}
  $f\in C^{1}(D,\mathbb{R})$. Dann gibt es auf jeder Strecke
  $[x_0,x]\subset D$ einen Punkt $\xi\in[x_0,x]$, so dass gilt
    \begin{equation*}
    f(x)-f(x_0) = \operatorname{grad} f(\xi)^{\top}(x-x_0)
    \end{equation*}
  \end{YetAnotherTheorem}

\end{document}