How make BBEdit Color Scheme follow system dark mode setting
BBEdit 13.0 introduced automatic color scheme switching based on the prevailing OS color scheme. (And it runs on macOS 10.14.2 and later, so you'll be able to use it.) Here's a screen shot of what it looks like in 13.1 (the current public release, as of when I wrote this answer.)
This is probably a simpler and more reliable solution than trying to automate it externally.