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Material theme https://jekyll-themes.com/matjek/
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Mimalistic with search https://jekyll-themes.com/millidocs/
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Nice green with search https://jekyll-themes.com/hcz/
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Simple theme https://github.com/arnp/herring-cove
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Highly customizeable and maintained http://jekyllthemes.org/themes/documentation-theme-jekyll/