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I am really not that good with responsive design, so this solution is not responsive haha, I know how to make media queries and that stuff but I really don't know like the fundamental rules of responsive design of web apps, where should I start?

Really not that good with coding responsive designs, where can I look for the best practices or tips to create good responsive websites?

Haven' really used Sass that much, i am using it correctly? Also I think the card proportions are not like perfect, how can I improve it?