The bar may be from being logged in with the admin bar present. Try adding this to your custom CSS:
.hero.admin-bar #header {
top: 0;
}
You can make the header a bit smaller too. Add this to the Custom CSS as well:
nav#primary-site-navigation {
margin-top: 0;
}
If you need to make it smaller than that, you can reduce the top and bottom padding of the menu items with this:
.sf-menu a {
padding-top: .75em;
padding-bottom: .75em;
}
The default is .75em so feel free to change that if you need to.
Also I did notice a change or two inside the style.css file. It’s perfectly fine to make changes there, just be aware that the theme won’t be able to be updated if changes are made inside it. They would be overwritten by the update. If you have a small number of css changes you can use the Custom CSS box. Just copy the style there from the style.css file and edit it there. If you have lots of changes then I would go with a child theme.
Hope that helps out. Have a great week,
Bill