Theorem environment under polyglossia

Question

How to set theorem header font to be normal+bold and theorem body font to be normal+italic in the example below? If I remove the lines starting with \usepackage{fontspec} and ending with \newfontfamily{\serbianfonttt}{DejaVu Serif}, then I get what I need.

\documentclass[10pt, b5paper]{book}

\usepackage{fontspec}
\usepackage{polyglossia}
\setmainlanguage[Script=Cyrillic]{serbian}
\setotherlanguage{english}

\newfontfamily{\serbianfont}[Mapping=ascii-to-serbian, Script=Cyrillic, Language=Serbian]{DejaVu Serif}
\newfontfamily{\serbianfonttt}{DejaVu Serif}

\usepackage{amssymb, amsmath} 


\usepackage{ntheorem}  
\usepackage[framemethod=default]{mdframed}   


\theoremheaderfont{\normalfont\bfseries}
\theorembodyfont{\slshape}
\theoremseparator{.}
\newtheorem{teorema}{Teorema}[section]

\begin{document}
    \begin{teorema}
        ssssssddd
    \end{teorema}
\end{document}

Answer 1

Set the main and mono font instead. If you want italics, use \itshape rather than \slshape.

\documentclass[10pt, b5paper]{book}

\usepackage{amssymb, amsmath} 

\usepackage{fontspec}
\usepackage{polyglossia}

\usepackage{ntheorem}  
\usepackage[framemethod=default]{mdframed}   

\setmainlanguage[Script=Cyrillic]{serbian}
\setotherlanguage{english}

\setmainfont{DejaVu Serif}[
  Mapping=ascii-to-serbian,
  Script=Cyrillic,
  Language=Serbian,
]
\setmonofont{DejaVu Serif}

\theoremheaderfont{\normalfont\bfseries\upshape}
\theorembodyfont{\itshape}
\theoremseparator{.}
\newtheorem{teorema}{Teorema}[section]

\begin{document}

\begin{teorema}
abcde fghijk lmnop qrstu vwxyz
č ć š ž dj 
\end{teorema}

\end{document}