greek text letters, e.g. \textsigma are not bold in titles and text

[TorkelE]

I use \usepackage{textgreek} to have greek letters in my text, however, this does not seem to be prop[erly bold in this template.

First, I create a normal document:

\documentclass{article}
\usepackage[utf8]{inputenc}
\usepackage{textgreek}

\begin{document}

\section{This is a section header with a \textsigma{} in it}

\textbf{This is a bold text with a \textsigma{} in it}

\end{document}

which yields this:

which looks fine.

Next, I add this within the chapters of my thesis (and adding \usepackage{textgreek} to the preamble)

\section{This is a section header with a \textsigma{} in it}

\textbf{This is a bold text with a \textsigma{} in it}

the result is:

Here, the sigmas are not bold. I do not know why, but it makes all titles with greek letters in them slightly weird.
(I can add them to the title within $\sigma$, but that makes the sigma in mathmode style, which it shouldn't be and which the rest of the title isn't in)