Blink the screen when your Mac needs to charge

1. Enable invert colors shortcut:
   System Preferences... -> Keyboard -> Accessibility -> Invert colors [x]
2. Allow macscript to modify Accessibility
   System Preferences... -> Security & Privacy -> Accessibility -> Privacy -> + -> osascript
3. Test that it's working from your shell:
   osascript -e "tell application \"System Events\"" -e "key code 28 using {command down, option down, control down}" -e "end tell"
4. Create the script
   sudo vim /etc/batteryScript.applescript
   
   set Cap to (do shell script "ioreg -w0 -l | grep ExternalChargeCapable")
   tell Cap to set {wallPower} to {last word of paragraph 1}
   if wallPower = "Yes" then
    return 0
   else
    set Cap to (do shell script "ioreg -wO -l | grep AppleRaw")
    tell Cap to set {Max, Available} to {last word of paragraph 2, last word of paragraph 1}
    set Pct to round (100 * Available / Max)
    if Pct <= 5 then
     tell application "System Events" key code 28 using {command down, option down, control down}
     end tell
     delay 1
     tell application "System Events"
      key code 28 using {command down, option down, control down}
     end tell
     if Pct <= 3 then
      delay 1
      tell application "System Events"
       key code 28 using {command down, option down, control down}
      end tell
      delay 1
      tell application "System Events"
       key code 28 using {command down, option down, control down}
      end tell
      delay 1
      tell application "System Events"
       key code 28 using {command down, option down, control down}
      end tell
      delay 1
      tell application "System Events"
       key code 28 using {command down, option down, control down}
      end tell
      if Pct <= 1 then
       display notification "Computer is about to shutdown!" with title "---=== WARNING ===---"
       beep 5
       do shell script "say -v \"Alex\" \"PLUG ME IN NOW\""
      end if
     end if
    end if
   end if
5. Configure script access:
   sudo chmod +rx /etc/batteryScript.applescript
6. Invoking script
   vim ~/Library/LaunchAgents/batteryAlert.plist
   
   <?xml version="1.0" encoding="UTF-8"?>
   <!DOCTYPE plist PUBLIC "-//Apple//DTD PLIST 1.0//EN" "http://www.apple.com/DTDs/PropertyList-1.0.dtd">
   <plist version="1.0">
    <dict>
     <key>KeepAlive</key>
     <false/>
     <key>Label</key>
     <string>batteryAlert</string>
     <key>LowPriorityIO</key>
     <true/>
     <key>ProgramArguments</key>
     <array>
      <string>/usr/bin/osascript</string>
      <string>/etc/batteryScript.applescript</string>
     </array>
     <key>RunAtLoad</key>
     <true/>
     <key>ServiceDescription</key>
     <string>Battery Alert</string>
     <key>StartInterval</key>
     <integer>30</integer>
    </dict>
   </plist>
   
   
7. Reboot the OS or manually load the launchd module.
   launchctl load ~/Library/LaunchAgents/batteryAlert.plist