Alternatively, would love to figure out how to remove the white border that surrounds photos embedded from elsewhere (eg Flickr).

I don't see that happening at the http://atthetreehou.se/ link you provided. Can you point me to a link showing the white border you want to remove?

Just looking for some code I can use to change or remove the gray colored shading in the blog's footer area.

I see you figured this out by using the following custom CSS:

#supplementary {
	background: #fff
}
#main {
	border-bottom: 18px solid #fff
}

Nice work!

Alternatively, would love to figure out how to remove the white border that surrounds photos embedded from elsewhere (eg Flickr).

Thanks.