That little silver pen others have mentioned works well for the very thin lines to the sides and the bottom edge. These edges are also fairly well protected and I have not had a problem with it rubbing off. These pens are ball-point with a liquid silver paint. Find them at art shops like Michael's in the US.
The top line is another matter. First, it is thicker than the other lines. second, being next to the switch it gets a lot of wear. That silver pen won't cut it.
I use a chrome spray paint and a clear acrylic top coat. Carefully mask off the line - spend as long as needed to get the masking perfect. Then I put the whole calculator in a small plastic bag that has a half inch wide slit cut for the trim to show through. I mask around the slit to protect the calculator from overspray. Then I give it a few coats of the chrome paint, let dry, then a few coats of clear acrylic.
After it has dried and the masking removed, there is typically a lip of paint where the masking edge was. This can be removed by running your fingernail across the edge.
This won't duplicate the chrome plate look. Despite the name of the paint, it's really a silver paint and so it has a matte texture, not the mirror finish of chrome. Still, it looks good IMHO.
Whether or not to do this is up for debate of course. I wouldn't do this on any highly collectible item. But I will often touch-up a run-of-the-mill Classic.
Sealed in bag, ready for painting:
HP-55 with restored chrome. Slightly underexposed to bring out the texture of the top line:
And the complete before-and-after restore: