Inspired by the predawn-ui theme for Atom and Sublime Text.
OS X Yosemite Dark Transparent
Install by downloading the theme and double clicking it.
Make sure you're running the latest pre-release (Updates tab in Alfred Preferences). Enable Window Blur in Appearance > Options - I set it to max.
Latest version on Github
Clean Dark Theme
Programmer's Solarized Dark
This is a theme using Ethan Schnoover's Solarized color scheme, with the proper background and foreground sets. Additionally, it defaults to Menlo, as we programmers like our monospaced fonts (easier to read). It uses yellow for the selected result highlight, with blue and magenta button accents. The fonts are on the big side, with the window being made a bit wider. Hope you like it!