problem with theorem, tikzpicture and center environment

If you try to place a theorem inside center environment, you will realize that it doesn't work.

Edit: I mean that the code:

\begin{center}
    \begin{theo}
    test
    \end{theo}
\end{center}

would result to a non-centered theorem.

So, my solution is a "hack" that just indenting your environment as much as needed:

\documentclass[a4paper,11pt,twoside]{book}
\usepackage[a4paper,left=2.5cm,right=2.5cm,top=2.5cm,bottom=2.5cm]{geometry}
\usepackage{amsthm}
\usepackage{lipsum}
\usepackage{environ}

\usepackage{tikz}
\usetikzlibrary{shapes,decorations} % Για φάνσι κουτακια

\theoremstyle{plain}
\newtheorem{theo}{Sometheorem}[chapter]

\tikzstyle{BoxDialog} = [draw=black, fill=white, very thick,
rectangle callout, rounded corners, densely dashed,callout relative pointer={(-0.4cm,-0.4cm)}, inner sep=4pt, inner ysep=8pt]
\tikzstyle{TitlTheo} =[fill=white, text=black]

\newsavebox{\theobox}
\newlength{\theoidentlength}
\NewEnviron{dialogtheo}[1]{
\savebox\theobox{\hbox{
        \begin{tikzpicture}
        \node [BoxDialog] (boxtheo){%
            \begin{minipage}{0.6\textwidth}
            \BODY
            \end{minipage}
        };
        \node[TitlTheo] at (boxtheo.north) {\textbf{#1}};
        \end{tikzpicture}}}
        \setlength\theoidentlength{\dimexpr(\textwidth-\wd\theobox)/2\relax}
        \noindent\hspace*{\theoidentlength}\usebox{\theobox}
}

\begin{document}
\lipsum[1]
\begin{dialogtheo}{title theorem}

\begin{theo}
    theorem theorem theorem theorem theorem theorem theorem theorem theorem theorem
\end{theo}
\end{dialogtheo}

\end{document}

PS: My solution is just an alternative [that I think could be useful in many cases if centering fails], and just added as such (an alternative) approach.

If your goal is to center the dialog bubble, instead of using the center environment, you should try to use \centering with a \par at the end of the content of your environment:

\NewEnviron{dialogtheo}[1]{
    \centering
    \begin{tikzpicture}
        \node[BoxDialog] (boxtheo){
            \begin{minipage}{0.9\textwidth}
                \BODY
            \end{minipage}
        };
        \node[TitlTheo] at (boxtheo.north) {\textbf{#1}};
    \end{tikzpicture}\par
}

The \par is important, else the bubble will not be centered at all.