One page project using HTML and CSS with Flexbox

As I haven't practiced CSS a lot, I was very unsure of dimensions of the elements. Somehow, tweaking the number of pixels doesn't seem right: if there're no dimensions specified in the design, you end up eyeballing everything (I would have preferred to have exact dimensions for padding and margins but maybe it's not how it's done?).

I used a red border on every element while editing CSS to be able to see where which element is. Is that considered good practice?