Firstly, i want to say how great i think Mantra is and the amount of customizable options are great!
For me there are a couple of things that would make it perfect.
Firstly, if the blog posts could sit in a small frame it would make things look a little smarter and more defined between the posts.
Secondly, I wonder if the header image could actually go across the whole of the page it is being viewed on. Just for the colour definition between the header and content below.
Small things, but could really make it that tiny bit smarter and easier to read.