Social Media Dashboard with theme switcher using HTML, SASS and JS

Hi everyone, this is the second project, i found hard make it responsive and i thought the dark theme was easier... It's very far from perfect, but i tried my best and eventually i'm quite satisfied because it's the first time i use sass. Some questions to anyone who has experience:

1. What do you think about my html and sass structure?

2. Any tips for getting better at how to approach responsive design since the beginning of a new project?

Thank you.