landing page use flexbox

What are you most proud of, and what would you do differently next time?

At first I don't know how to use flexbox,

• why the picture not in the center?
• why the text not center align?

which css selector I can control and solve the problem, I have no idea. Just try different way, at last I achieve to make all center, although it is ugly code.

so I search in codepen and find other solution, and I learn how to write css code, and how css property make affect, such as box-sizing transform