Thicker line for minus sign and plus sign

Here's one way to do it:

documentclass{article}
usepackage{bm} % for the boldsymbol
begin{document}

$boldsymbol{+} $
$+ $
$boldsymbol{-}$
$ - $
newcommand{pl}{boldsymbol{+}}
$pl $
newcommand{mi}{boldsymbol{-} mathrel{mkern -16mu} boldsymbol{-}}
$mi $

% By increasing 'mu' you can shorten the minus sign.
end{document}

I used How can I get bold math symbols? for obtaining thicker math symbols and I also used Joining characters in latex with no visible space between them to extend the minus sign (although you can slightly tell it has been overlapped, which doesn't seem perfect). The new command definitions make it more convenient for you to use.