I want to use the following lines to swtich between two themes on different terminal emulators.

zplug "robbyrussell/oh-my-zsh", use:themes/ys.zsh-theme, as:theme, if:'[[ "$foobar" == bad ]]'
zplug "sindresorhus/pure", from:github, use:pure.zsh, as:theme, if:'[[ "$foobar" == good ]]'

But I got a weird behavior:
Whatever the foobar variable is, the zsh starts with the same theme. The theme depends on the environment I run zsh for the first time after I modify the .zplug_loadfile.
That is, when I change my zplug configuration file, and restart zsh, and if at this time, foobar == bad, the theme will be ys. This is of no problem. But then, whenever when I open a new zsh, and with foobar == good, the theme will still be ys. And it will never change again.

Is there something wrong with my concept? I've searched through all the documentations about if tag, but found nothing about this problem.