mobile first approach with flexbox

whew, this is the most challenge page I have done so far. I am glad that I finished this. I have some questions related to how to organize your css. Lets say you have a section that has padding/margin, font-size, background, border, flex/grid and etc. How would you organize them within the section? Which one should put top to bottom? ie. padding/margin, background, border, flex/grid then font-size?

any feedbacks are appreciated!