How to shrink the TitleBars in macOS Big Sur 

Notes

• Requires turning off SIP
• Requires MacForge

How to:

• Download and setup MacForge
• Search for the plugin MEMiniMe
  
• Install MEMiniMe
• Enjoy mini titlebars

Alternatively:

Alliteratively if you just want to have the titlebar and toolbar separated like they were in previous versions of macOS you can run this simple command in terminal. Once you've run the command you'll need to log out and back in to see the changes take effect.

• Enable defaults write -g NSWindowSupportsAutomaticInlineTitle -bool false

• Disable defaults delete -g NSWindowSupportsAutomaticInlineTitle 

And results look like this: