The upright theta in these examples is not typographically correct.
Here's the problem. "Standard" Computer Modern does not "come" with upright greek letters, but with a slanted letter. The textgreek
package provides a way around that problem, by using the cbgreek
font, and defining \texttheta
and so forth to "borrow" from them.
And that is fine ... but the cbgreek
font uses the style of theta that looks like example (a) in the illustration below, not (b). These are not different letters, but variant forms of the theta (though unicode assigns separate numbers so that both can be present in a single font).

What then happens is that, in order to try to provide a non-script version of theta, textgreek
is "borrowing" from either the Euler font (which is what is shown in your first example) or -- I think -- the symbol font (which occurs in your second, because you select Helvetica which textgreek
"maps" to symbol).
Since these thetas were designed for quite different fonts they are a very poor match for computer modern -- each has a quite different design, weight and so forth.
Here are a few ideas:
- Put up with a bad looking straight theta (OK, pehaps, if you only use it in one place).
- Accept the scriptstyle theta that is part of the
cbgreek
font, and load textgreek
and use \texttheta
.
- Take a quite different approach to greek letters outside maths, for instance by using XeLaTeX which will enable you to use unicode and any font you choose (though you will then have to consider matching math fonts).
- Redefine
\straighttheta
. For instance, if you load the lgreek
package (which has a different font, with a straight theta) you can redefine the \straighttheta
command of textgreek
to be {\begin{greek}j\end{greek}}
However, although that produces a somewhat better match (I think) in some respects, it has its own problems in your example, because there is no slanted version of the lgreek
font. But it's OK with math, because the math letters get taken from Computer Modern. The picture below shows an lgreek
based theta in text alongside CM theta in math.
