Go to the application in Finder, right-click it, and click “Show package contents”. Go inside the folder “Contents”, there you will find a file called info.plist, which is the file we are going to edit.
First make a backup in case you want to easily revert or if something goes wrong. If you have Xcode, open the file with Xcode, if not, open it with TextEdit. The reason I say TextEdit is because some plaintext editors cannot handle .plist files.
With Xcode
Hover the mouse over any key and click the plus to create a new key. Name it NSUIElement, and change the value to 1.
With TextEdit
Add:
<key>NSUIElement</key>
<string>1</string>
to the second last line, just before </dict> and </plist>
Save the file, and restart the application. It will now run in the background.