The best alternatives to Antetype: Advanced Prototyping are Froala, lstore.graphic, and Maze. If these 3 options don't work for you, we've listed a few more alternatives below.
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Froala builds editing software for fast development with the better developer and user experience in mind. With over 35+ plugins and 15+ natively supported frameworks, you can plug Froala and be on your way.
Chameleon is a macOS app that gives quick access to changing the interface, accent and highlight colors from the menu bar. It has a very simple interface, if you've used the System Preferences panel to make any changes then it should look really familiar.