Alfred Theme inspired by a random Terminal.app color scheme I once downloaded, called DotGov
[UPDATED - includes link for Alfred 3 theme]
A dark theme with a bold selected text background inspired by HTML5Boilerplate, which sadly no longer uses hot pink to let you know you've got some text selected. Selected items have a darker background with brighter, more saturated text to enhance contrast.
Packal doesn't like the base64-encoded Alfred 3 version of the theme (that now includes all the settings, including blur), here's theme share link (trust this screenshot though – the alfredapp share page doesn't currently show off the selection highlight). There's also a version with San Francisco set as the main font.
For those who are sticking with Alfred 2, I recommend using a blurred background to help keep busy backgrounds from making text hard to read; since that's a setting that I can't include in the theme, you can adjust the blur; I use the 4th strength available (out of 6).
Clean Dark Theme
An elegant, dark, and minimalistic theme focusing on selection emphasis and icon contrast for all workflow icons.
This is my preferred theme while being productive with Alfred.