[Tex/LaTex] Make math symbol bold and non-italic simultaneously in latex

Question

How to make a math symbol bold as well as non-italic in the math-mode of latex. I have been trying to both bold and non-italicize x and \theta using the following minimum-working-example. x can be bold or non-italicized but cannot be both. While \theta can only be bold but neither non-italicized nor both. What is wrong with the code and how should one do it?

\documentclass{book}
\usepackage{amsmath}
\usepackage{bm}

\begin{document}
    \begin{equation*}
    x ~ \bm{x} ~ {\rm x} ~ \bm{\rm x}
\end{equation*}

\begin{equation*}
    \theta ~ \bm{\theta} ~ {\rm \theta} ~ \bm{\rm \theta}
\end{equation*}

\end{document}

Answer 1

A simple \mathbf and package upgreek for upright Greek letters (see Bernard's comment) are your friends:

\documentclass{article}
\usepackage{upgreek}
\begin{document}
\[
  (x, \theta) \Rightarrow (\mathbf{x}, \mathbf{\uptheta})
\]
\end{document}