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Thank you, Stefan. :D

I have tried this before, but I forget to add it in \tcbuserlibrary, sorry my fault. :) \tcbuselibrary{theorems, skins, breakable}

So now it works, but I get a new problem.
\newtcbtheorem[use counter from = MyDef, number within = chapter, Crefname={Satz}{Satz}]{MySatz ...

Hello everybody,

im working on my bachelor's degree and have a problem with my theorems.

A friend of mine told me i should use "\newtcbtheorem".

So i made some for every case i need.

\newtcbtheorem[number within = chapter, Crefname={Definition}{Definition} ]{MyDef}{Definition}%
{fonttitle ...