Personally, what I’d do is download and try a few of the themes found in this thread:
If you can find one that has a line-numbering font that seems to not be common to most of the interface, that would mean that, yes, it can be done.
(Although not finding one wouldn’t mean it can’t.)
Otherwise, as far as the default theme is concerned, no, it can’t be done ; this is not a setting that one may tweak in the options/appearance.
Meaning that in order to achieve that – if even doable – you’d have to design yourself a new theme. Even should it only have this for a difference.
[EDIT] I just saw that @AntoniDol liked your post, but without providing an how to.
To be honest, if he doesn’t know, I can’t think of any mortal user who does.
BTW: I use line numbering a lot. As in, almost 90% of the time. And one thing I can tell you is that, since you’ll actually only look at it when needed, looking for a number, it ain’t that bad. You should expect to get used to it quite rapidly. It may look faint for now, but that’s likely because you are looking at it in a general way ; comparing it to other elements of the interface, perhaps.
When you are actually looking for a number, they are easy enough to read.
Not sure I am being that clear about what I mean, but give it time, you’ll get my point.
Else yes, I’ll agree with you, there is plenty of room for improvement. It certainly wouldn’t hurt for them to be somewhat more visible.