Update to the visual theme of the blog

After nearly two years and over 300 posts, I decided to make some minor changes to the theme & background of the blog. The most obvious one is the the updated background image & color (reproduced here at right).

Since the original background image (a radial tree with a multicolored stochastic mapping) was created before phytools had any graphical functions (and, in fact, before the package formally existed), I thought the update was overdue. The new images were created using densityMap and contMap with some very minor post-hoc editing.