Responsive Design using the CSS Flexbox and the good'ol Vanilla CSS

Hi.

• consider elements that give your site meaning. Elements such as main , header, footer are more important than divs.
• the first heading element of a site has to be an h1, and the headings has to flow in order without skipping any headings
• you can take another screenshot to reflect changes that you may have made.If you are not a Pro member - screenshots are limited

I'm by no means an experienced front-end developer but I can share with you some tips:

• If you have a value that is repeated multiple times in your styles (colors in your case), put it in a variable and call it every time you need it. Besides, whenever you want to change that color, you only change that variable's value instead of going through all your code.
• Avoid using uppercase when naming selectors.
• For formatting your code, I recommend Prettier extension on VSCode if you're not using it already.

Other than that, your code looks clean already.