Sidebar 100% height of content section

I’ve altered the stylesheet and .php files as desired to change the F2 theme somewhat.

You should not modify the theme files directly, but instead create a child theme; you don’t want to lose your modifications in case the theme gets updated because of a security patch or feature updates. You don’t want to end up like this individual.

What I would do is make it appear as if the sidebar extends to the bottom by changing the color of the main DIV to the same color as the sidebar, and make the primary DIV that contains the blog post white. Try adding this CSS to either the Custom CSS option of JetPack, or to the end of your child theme’s style.css file (if you do go ahead & create a child theme):

/* Change background color of #main DIV to same color as sidebar */
#main {
	background: #eee;
}
/* Set background of #primary DIV to white so it stays white */
#primary {
	background: #fff;
}
/* Remove box shadow from sidebar */
.medium-sidebar.one-sidebar-right #secondary {
	-webkit-box-shadow: none;
	-moz-box-shadow: none;
	box-shadow: none;
	border-bottom-left-radius: 0;
}