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I've got it ^^ As you said I set the first argument to 10pt. Thanks

I mean that space.

Thanks to you Johannes, again. I know what you mean with the roman letters but for that one work, I need it. Know I'm happy with the result. My last question is, how to set a bit more space between the black border of the theorem and the headline? Just a bit, so that it's the same as it is between ...

Thank you Johannes! Is it possible to use one counter for different theorems (corollary, theorem, definition) but with one counter and roman numbers in the headline?

Hello,

I'm trying to optimize my math article. I wrote enviroments for definitions, theorems, and so on...
I like to shade the backround of a theorem or definiton. But I'm not able to set the headline in the shaded box. I uploaded a picture with an example.

\newcounter{number}
\newenvironment ...