I saw some other Yosemite's Spotlight-style themes but this one is closer.
Yosemite's Spotlight does a weird thing where it subtly blends the user's desktop background with the white, then applies a blurred opacity against whatever is currently behind it.
closest we can get is
defaults write com.runningwithcrayons.Alfred-2 experimentalBlur -int 15
in the terminal (see and a theme like this.
Here is Yosemite spotlight:
And here Alexandria:
and here is a link: https://snaek.org/s/goyun/Alexandria.alfredappearance