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}

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