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To learn how to create your own theme (or modify an existing one), check out the Dev docs: https://getpublii.com/dev/. I’ve been customizing Publii themes (mainly Mercury) over the last year, and the Dev Docs have been incredibly helpful. I’d also check out this tutorial for an overview of how someone created their own Theme, check out this tutorial: https://github.com/SimonPadbury/Publii-Bootstrap-Starter/tree/master/TUTORIALS
For plugins: this feature is still new. From what I read on another thread, documentation for that is coming. In the meantime, I would reach out to other users who’ve made their own plugins already: