fylo-darm-theme-landing-page using css and flexbox

It's a very good project for testing flexbox and grid layout. I am kind of scared of complex layout of webpage, do you guys have any good ideas to improve the skills to do the layout. I am still headache about a complex layout mixed with responsive webpage and mobile first design. Learning a lot by doing this project though I know there are still so much problems there. Welcome anyone give any feedback, I am just a beginner of beginner on layout. Thank you.