Don't intend to be harsh but,You took a cheap way out with a Masonite door from HD and their installers .I understand the job should have still been done correctly but what can you really expect?

I see a lot of splits and cracks in your floor continuing thru the riser and tread on your first step.How would installing a door do that?

Did you use their "hom" services? They warrant their work. My warranty was one year. You should have a rep/scheduler that you can call. Also they want the installation to be rated. They want 10 out of 10. Anything less they will want to know why.

If you used their directory of installers, I was told that they will also stand behind the work of those installers. In that case, call the manager of the store that you bought the door from.

As far as the threshhold color is concerned, my door came with a black fiberglass threshhold too. That was the only choice for that door and I knew it going in because I had seen a floor model. Sometimes the in-store associates that configure the doors don't know all the details (and/or are overworked) and give you a printout that is hard to understand. I examine it thoroughly and ask questions about every item on that list that looks like a color or material. I also back-seat drive the configuration process on the computer. They don't care for that, but I am nice to them in other ways and afterward so they forgive me for it. I think the more questions you ask before the order, the more they realize that you will have an issue if something unexpected comes up, so they should not resist helping you get it right. I spent hours talking to the door consultants. It's too late for this installation, but if you do another door, be thorough up front so there are no (or few) surprises and be ready to address mutually beneficial solutions to any problems that come up during installation. Any remodel is rarely perfect. You have to help them help you to your satisfaction.

I think the black strip looks fine. There are always other black finishes in a home so it does not look out of place to me.

Can you use wood filler on the floor?

To me, the most important things about a door installation are that they are plumb, level, fit, aligned, sealed and operate properly. If you watched them do the installation, you would know how many steps and checks and adjustments they do. My installers did not do a perfect job, but considering how complex the job was, they did a really good job. They scratched my composite deck taking the old doors out and got smudges on my new paint. The deck scratches can be sanded and the paint had to be touched up anyway since new casing and caulk was installed. So I was just happy that my new door worked properly. I did ask for free touchup paint for the door since it was not perfect. Shipping and handling creates imperfections but as long as I can fix it, I can deal.

I opened a claim with home depot 2 weeks ago, they are working on it. The door installer wants to fix it, but I don't agree fixing it without starting from the bull nose of the stairs. just cutting a piece of floor and sticking a new one is not a solution. I will wait for a resolution from home depot. Hopefully will send somebody knowledgeable or they will pay my floor guy to fix it. Thanks for advice.