I've written a command-line script that will automatically change your Alfred theme at sunrise and sunset. It's kind of a companion to F.lux and its option to turn on Yosemite's dark theme when it gets dark. It requires the pytz and astral Python libraries. Save the script somewhere and run it in Terminal:
python toggle_alfred_theme.py --dark 'My dark theme' --light 'My light theme'
The script will change the current theme depending on the time of day and then schedule itself to be called every sunrise and sunset (or when the computer boots/wakes) to change Alfred's theme.