I've seen examples in some themes of text featuring a gradient as their fill color instead of a single sold value, but I've not been able to find where one would be able to implement such a styling in the theme editor. Is there an option I'm not seeing?

I don't think we support gradients as font colors, and I also don't see how someone could do it even with a script.
I could think about enabling it though, have to see how to implement it properly then.

For full disclosure, this is one of the examples I'm referencing, from iRacing's ATVO theme, on the top line. There are similar text treatments elsewhere in the theme, but this is the clearest instance.

https://imgur.com/YqvS48l.png

That's what we call a custom theme. It is not made with the Theme Editor. Such themes give us full freedom to make whatever design we want, while the Theme Editor is obviously more limited.