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Bulma is a free, open-source CSS framework based on Flexbox and used by more than 200,000 developers. It's the easiest to learn due to its natural naming of classes and a simple grid system. You can customize colors, breakpoints, type, and custom imports to decrease the file size. You need a Sass workflow to customize it using Variables which is only possible if you're using the node-sass sass-loader. You'll not be able to customize anything in this way, because of Bulma's nature.