The best alternatives to Sweet Themes are Slack Themes, Meteor, and PokéSlack. If these 3 options don't work for you, we've listed a few more alternatives below.
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Haggle — A CTA buyers want to click, when they aren't ready to demo
Meteor is an open source platform for web, mobile, and desktop used by over half a million developers around the globe to make shipping javascript applications simple, efficient, and scalable.