Wow. It is a plus sign. Barely.
I'm using Chrome on Windows 10 right now, Ros. I can see it...with magnification and with reading glasses. What I'm also seeing is that, if I've upvoted a question, in the listing the grayed-out downvote arrow is gone. I assume it may work the same way if I'd downvoted a question, but to my knowledge I never have.
Just a general BTW for everyone: on many platforms, if you use a mouse with a scroll-wheel, you can hold down the "Ctrl" key (Command key on a Mac) and then use the scroll-wheel to zoom the Web Browser display in or out. Max zoom level on most platforms is, I believe, 500%. Comes in handy. The scroll-wheel without the Ctrl key works the same way when viewing Google Maps and some other applications.
Edit: Yeppers. What Bob said. Looks just like that. But if you go in really tight, it is a plus-sign, at least on my display. But yeah, it's functionally a dot.