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HackerThemes - Bootstrap 4 themes and tools for web developers
PlainAdmin - PlainAdmin is an Open-source freemium Bootstrap 5 based vanilla JS multipurpose admin template comes with - all essential dashboard components, pages, UI elements, charts, graphs, libraries and everything you may need for a data-rich backends.
Bootstrap 4 Cheat Sheet - An interactive Bootstrap 4 cheat sheet
ArchitectUI - Modern dashboard template for bootstrap 4