Give your submit button display:block and margin:0 auto;
Simply give it text-align:center
Looks like your logo is taking too much space. Consider shrinking that first. By default, ul list stacks in a column. There are few ways to change this to be displayed vertically. The current best practice would be using flexbox. flexbox by default stacks things in a row. You can google search more about it and learn it but to spill some beans for you, you can probably achieve that by giving your ul element display:flex;
Here is a good article to read on flexbox.
Consider re-taking FCC’s responsive web design certificate to remind yourself It’s always good to go back.
@shimphillip , Thank you for your assistance and I apologize for not replying sooner. I’ve found myself suddenly swamped at my business here in Japan. I’ve been forced to spend a lot of time offline away from coding as I’m getting a new teacher up to speed and interviewing with a great company for a remote work position.
I will definitely give your suggestions a try as soon as I can.