Most of it is taken from this gist. I just tweaked the script to check the right theme at startup in case the theme change occured when iTerm2 was closed.
Make sure the theme is the same as the themes you have installed (Preferences > Profiles > Colors > Color Presets...). I personally use Solarized Dark and Solarized Light
From someone on the linked gist:
First-time installation steps for me:
- Download the script from GitHub by right-clicking on Raw button and save as...
- copy the script to
$HOME/Library/Application Support/iTerm2/Scripts/AutoLaunch- create AutoLaunch folder if it does not exist
- go to iTerm2 > Scripts > AutoLaunch