MWE:
\documentclass{beamer}
% Custom theme from https://bitbucket.org/marczellm/beamerports/src/30de28e687e2dfad6ebe0765d2eb622ed12fdbfe/beamerthemeKlope.sty?at=master
% It overwrites the default list style (see screenshot below)
\usepackage{../marczellm-beamerports-ad2150496d62/beamerthemeKlope}
\begin{document}
\begin{frame}
\begin{enumerate}
\item A
\item B
\item C
\end{enumerate}
\end{frame}
\end{document}
I wish to change the indentation of item C. The closest result I achieved was by using \item \hspace{-4em}C. Obviously the number 3 has also to be moved to the left.

I also read about leftmargin in conjunction with itemize. Unfortunately, it does
a) not work with enumerate: \begin{enumerate}[leftmargin=-.5in] ("Something's wrong--perhaps a missing \item.")
b) and I don't see a way of assigning it to specific items. \item[leftmargin=-.5in] gives weird results.
@Steven B. Segletes' (now deleted) answer: I am actually using Beamer and a custom theme (this one). It seems that overridden styles are not taken into consideration with your code:

