ntheorem: Set normalfont to Theorem-Note

Question

What is the key to set normalfont to the theorem-note and its brackets and its punct?

\documentclass{scrreprt} 
\usepackage{ntheorem}
\theoremseparator{.} 
\theorembodyfont{\upshape}
%\theoremheaderfont{\normalfont}
\newtheorem{defi}{Definition}

\begin{document}
\begin{defi}[Normalfont Here]
Blablabla
\end{defi}
\end{document}

Answer 1

You didn't indicate which of ntheorem's pre-defined theorem styles you employ -- plain, break, change, break, margin, marginbreak, nonumberplain, nonumberbreak, empty, and emptybreak -- so I'll assume you're fine with using the default style, viz., plain.

If this assumption is appropriate, just add the following instructions to the preamble (after loading ntheorem, obviously):

\makeatletter
\renewtheoremstyle{plain}%
  {\item[\hskip\labelsep \theorem@headerfont ##1\ ##2\theorem@separator]}%
  {\item[\hskip\labelsep \theorem@headerfont ##1\ ##2 \mdseries (##3)\theorem@separator]}
\makeatother

Just in case you're curious, the default for the fourth line in the preceding chunk is

  {\item[\hskip\labelsep \theorem@headerfont ##1\ ##2\ (##3)\theorem@separator]}

Thus, the only change consists of the insertion of \mdseries immediately before (##3).