How to decrease space between menu/header & menu/posts?

.bg-image-header .site-branding, 
.home-header-bg .site-branding {
    padding-bottom: 0;
}

if it does not work put !important after the 0 and before the ;

Hope it helps

THANKS! Is there a way to eliminate the space under the menu & before the 1st blog post? There is supposed to be the “Blog Title/Description” there, but I don’t want to use it.

Depending on what you want to do there are a couple of ways:

.home .blog_page_titles {
    display: none;
}

or

.blog .blog_page_titles {
    margin-bottom: 0;
}

This solutions assume that this is only for the home page and/or for the blog page, in your case right now is both since you are displaying the blog on the home page. The bottom line is that you can exchange .home and .blog in this particular case but if you go for the display: none; you gain more space than just reducing to 0 the margin-bottom.
If solution 2 does not work try to add !important between 0 and ;

Hope it helps
G.