Apex launcher is a cool launcher for the Android Phones. So if you are an Apex user, you can create a theme for the Apex launcher so easily. First you need to have setup an Android development environment How To Setup Android Development Environment on Windows
Now you can read the further steps very detailed on the XDA Developers, where you can download a sample and can edit it.
You will have to find package names and activity names for creating the themes. You can use Search Android Activities to get these details.
I have created a theme
If you like the above theme you can find on the play store here