documentclass{beamer}
usepackage[latin1]{inputenc}
usepackage[T1]{fontenc}
usepackage{amsmath,amssymb,amsfonts} %math
usepackage{graphicx}
usepackage{pifont} %bouni
%--------------------
usepackage[most]{tcolorbox}
newtcolorbox{mybox}[2][]{%
colback=bg,
colframe=blue!75!black,
fonttitle=bfseries,
coltitle=blue!75!black,
colbacktitle=bg,
enhanced,
attach boxed title to top left={yshift=-1.2mm, xshift=2mm},
title=#2,
#1}

usepackage{color} %couleur

usetheme{Warsaw}

begin{document}
begin{frame}{}
begin{mybox}
[colframe=blue!75!black,coltitle=black!75!black,colbacktitle=white!90!blue,
]{ding{43} Definition}
Let ${bf T} = (T_1,cdots, T_d)$ be $d$-tuple of operators in $mathcal{B}
 (mathcal{H}).$ The joint numerical range of ${bf T}$ is the set $JtW({bf
 T})$ of  $mathbb{C}^d$  such
 that
 $$JtW({bf T})={(langle T_1 x; |;xrangle,langle T_1 x;
 |;xrangle,cdots,langle T_d x
  |;xrangle);:x in mathcal{H},;;|x|=1}.$$
end{mybox}
end{frame}
end{document}

documentclass{beamer}
usepackage[latin1]{inputenc}
usepackage[T1]{fontenc}
%-----
usepackage{amsmath,amssymb,amsfonts} %math
usepackage{graphicx}
usepackage{pifont} %bouni
%--------------------
usepackage[most]{tcolorbox}
newtcolorbox{mybox}[2][]{%
colback=bg,
colframe=blue!75!black,
fonttitle=bfseries,
coltitle=blue!75!black,
colbacktitle=bg,
enhanced,
attach boxed title to top left={yshift=-1.2mm, xshift=2mm},
title=#2,
#1}

usepackage{color} %couleur

usetheme{Warsaw}

begin{document}
begin{frame}{}
    begin{mybox}
        [oversize, colframe=blue!75!black,coltitle=black!75!black,colbacktitle=white!90!blue,
        ]{ding{43} Definition}
        Let ${bf T} = (T_1,cdots, T_d)$ be $d$-tuple of operators in $mathcal{B}
        (mathcal{H}).$ The joint numerical range of ${bf T}$ is the set $JtW({bf
        T})$ of  $mathbb{C}^d$  such
        that
        $$JtW({bf T})={(langle T_1 x; |;xrangle,langle T_1 x;
        |;xrangle,cdots,langle T_d x
        |;xrangle);:x in mathcal{H},;;|x|=1}.$$
    end{mybox}

end{frame}
end{document}

documentclass{beamer}

usepackage[latin1]{inputenc}
usepackage[T1]{fontenc}
usepackage{amsmath,amssymb,amsfonts} %math
usepackage{graphicx}
usepackage{pifont} %bouni
usepackage[most]{tcolorbox}
usepackage{color} %couleur

usetheme{Warsaw}

newtcolorbox{mybox}[2][]{%
        width=1.05textwidth,
        left skip=-0.5cm,
        colback=bg,
        colframe=blue!75!black,
        fonttitle=bfseries,
        coltitle=blue!75!black,
        colbacktitle=bg,
        enhanced,
        attach boxed title to top left={yshift=-1.2mm, xshift=2mm},
        title=#2,
        #1}

begin{document}

begin{frame}{}
begin{mybox}[%
        colframe=blue!75!black,
        coltitle=black!75!black,
        colbacktitle=white!90!blue,
        ] {ding{43} Definition}

Let ${bf T} = (T_1,cdots, T_d)$ be $d$-tuple of operators in $mathcal{B} (mathcal{H}).$ The joint numerical range of ${bf T}$ is the set $JtW({bf T})$ of $mathbb{C}^d$ such that

[
JtW({bf T})={(langle T_1 x|xrangle,langle T_1 x|xrangle,cdots,langle T_d x|xrangle):x in mathcal{H},|x|=1}.
]

end{mybox}
end{frame}

end{document}