The treads aren't stain-ready lumber. They're inexpensive knotty wood meant to be covered up. The least expensive option is to recarpet them. The balusters were done in an inexpensive manner in the '80s or '90s most likely. I had a railing like that in my house that I replaced two years ago. If you really want to update your house you need to completely redo the stairs. That would be very expensive since you say you have another set like this as well. You're probably already spending a fair amount on the new flooring, so I expect you don't want to undertake a major project with the stairs. I hope your new floors don't end up making the stairs look worse. Maybe do the stairs in a couple years, or the next time the carpet wears out?

We have that same railing. You have my sympathy. Our home was built in the 70s. Really not a fan over all of it. Our stairs are carpeted. Not worth the $$$$ to start over for us.

Paint everything either a glossy white or black and then do carpet "mats", on the treads.

You just started another thread about some quirky interior details found in this house. If you separate all the areas/rooms you are going to get terrible design suggestions and won't end up with a cohesive vision for the house. Each room can't be renovated in a vacuum.

Since you're adding hardwood (hopefully in the carpeted area in the back), I'd match the staircase to the new floors. Check local code for staircase reno requirements (width, tread, riser dimensions) though-it can be a can of worms. If the railings won't be updated, I'd just replace with full carpet again since those treads are meant to be covered.

Given that you are looking at the expense of upgrading two sets of stairs, I would think about eliminating one set--greatly cutting the cost and direct that savings to rebuilding a single set. And gain floorspace. Since you are replacing the flooring now, seems like a no-brainer to do this now.

Why are there 2 sets of stairs ? I have to agree if both go to the same place upstirs I would eliminate one but we need more info. I think you might be able to cap the old treads but carpet is probably the cheapest choice . The railings and spindles very much the style the house is so what else have you done ?I see very different style in the divider so we need some info.I do agree keep all this stuff relating to thise home on one post or you and we will go crazy trying to even begin to help. BTW that goofy stuff on the upper floor has to go I think before doing anything including the floors you need to stop and figure out all of this .

Keep the rail and posts the wood, and paint the spindles and the treads in a black. You will need a hard wearing paint to stand up to abrasion, so you may want to just sand those treads down and dye them black instead. Then clear coat them.

Love the black staircase idea suggested by Sabrina Alfin Interiors, here's one in a similar style to yours:

Those are truly awful. Apparently the builder had his deck man move inside. Rebuild please.