Google "Mirror Mate" and you can see a product you can buy to do this.
But like other people on this forum, the lightbulb will probably go on immediately and you can figure out how to make this yourself less expensively and matching your tastes exactly.

(PS: I have the same mirror you do, in 2 bathrooms. I'm scared to take them down because (1) I don't know how much adhesive was used in the back and (2) I don't know if I want to see the wall condition and a big, huge rectangle of plain drywall instead of even a base-color paint!)

Thanks for the tip. I'll check it out.

I'm afraid to take mine down too. I haven't painted yet, but have already decided that I will paint around the mirror. At least yours in up with adhesive. Mine only has the little plasic hooks and it actully bows out a bit in the center. I'm scared to death that it will break.... I'm trusting that our builder know what he was doing (famous last words, right?)

I had someone make a frame for our mirror using fluted trim. I guess it's about 3" wide and instead of mitering the corners, I just used those corner rosette blocks. It looks pretty good. I have seen big mirrors such as yours with smaller frames (maybe about 24 x 30") mounted over each sink and that looked good, too. I have one word of caution if you do a painted frame and that is to make sure to paint the back of the frame black. I told the guy who built/installed my frame to be sure to do this so you couldn't see the wood on the back and he didn't listen. Sure enough, you can see the reflection of the back side.

Thanks for the tip Betsy. I really like what I saw on Mirror Mate, but I think this is something we can make ourselves and save a lot of money.

Your can also get window film that looks etched and make a frame of it. I have no clearance behind my faucets for a frame and that's what I am doing.

This isn't a great photo, but I bought a bag of the flat backed florist marbles at Michaels, got out my hot glue gun, and ringed the mirror with them. It remind me of the antique Murano mirrors. You could use any color; I used clear.

Or, build a frame for your mirror from decorative molding, or even from picture frame molding (though the latter could get expensive).

Or, you could take it down (pay someone to do it if you're scared) and replace it with a mirror you like.

Oceanna, I love that! I have a big plain mirror myself and I think that looks great! How many marbles did it take?

If you google 'how to frame a bathroom mirror" many blogs will come up showing the process....many are very detailed with great photos.

One thing you need to remember is ti be sure to paint (black) the Back of the moulding...because the back will reflect out in the mirror.