Here are some free resources for themes on HTML5 and CSS3. I think that some of these sites offer Bootstrap themes as well but not all of them are Bootstrap, they also have vanilla HTML5 and CSS3.
• HTML5 UP
• FreeHTML5.co – Free Website Templates, Free HTML5 Templates Using Bootstrap Framework
• 400+ Free Website Templates | HTML & Bootstrap 2019 – Colorlib
• ZEROTHEME – 120+ Best Free Responsive Html5 Website Templates
• Free Website Templates | HTML5 Website Templates | Styleshout
• HTML5xCSS3 – Free Responsive Html5 and Css3 Templates
• 20+ Best Free Bootstrap Admin Templates 2018 – aThemes
• Download Free HTML eCommerce Templates for Online Shopping Websites
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