Some of you are asking the following question: if I use the iOS 12 SDK, will my app run only on devices running iOS 12? No. If you want your Mac app to look good in dark mode, you have to build it with the 10.14 SDK. The macOS 10.14 SDK includes support for dark mode.

By using the latest SDK, your app can take advantage of the features Apple added in the latest version of iOS and macOS. The SDK contains the features your app can use. Xcode 10 ships with the iOS 12 and macOS 10.14 SDKs. Why should you use the most recent Xcode version? Each version of Xcode ships with a SDK (Software Development Kit) for iOS and macOS. The site Xcode Releases has a list of every Xcode release and the operating system requirements. Currently the most recent version of Xcode is version 10.1, which runs on macOS 10.13 and 10.14. You should use the most recent version of Xcode that will run on your Mac. Short version: use the latest version of Xcode and set the deployment target for the project to the earliest version of iOS or macOS you want to support. Answering what is basically the same question twice in one week is a sign that people are not sure how to make their apps run on older versions of iOS and macOS. I recently answered a question from someone who wanted to know what version of Xcode and Swift to use to develop an app for iOS 9 and another question from someone who wanted to know what version of Xcode to use to develop an app to run on macOS 10.7.

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