WET theme: shrink logo and add top/bottom margin on splash

Visual tweak to the splash page:

* Adds top/bottom margin around logo and buttons.
* Reduces size of the WET logo.

CSS has been added as an inline `<style>` block because I wasn't sure if 4.0 was going to have utility margin classes.

![wet-splash](https://f.cloud.github.com/assets/2110107/1484372/389b202a-4706-11e3-8a29-6cab7bb155ba.png)