The issue here is complicated by the fact that `\mathbf`

(the
command for setting bold *text* in TeX maths) affects a select
few mathematical “symbols” (the uppercase Greek letters).

In the default configuration, lower-case Greek letters behave differently from upper-case Greek letters (the lower-case greek letters are in the maths fonts, while the upper-case letters are in the original (OT1-encoded) text fonts).

The Plain TeX solution *does* work, in a limited way; you set a
maths *style*, before you start an equation; thus

- snippet.latex
{\boldmath$\theta$}

does the job, but `\boldmath`

may not be used in maths mode. As a
result, this solution requires that you embed single bold characters
in a text box:

- snippet.latex
`$... \mbox{\boldmath$\theta$} ...$`

which then causes problems in superscripts, etc. With amsmath loaded,

- snippet.latex
`$... \text{\boldmath$\theta$} ...$`

does the trick (and is less bad in regard to superscripts, etc), but is an unsatisfactory solution, too.

These problems may be addressed by using a bold mathematics package.

- The bm package, which is part of the LaTeX tools distribution, defines a command
`\bm`

which may be used anywhere in maths mode. - The amsbsy package (which is part of AMSLaTeX) defines a command
`\boldsymbol`

, which (though slightly less comprehensive than`\bm`

) covers almost all common cases.

All these solutions apply to all mathematical symbols, not merely Greek letters.