Skilled eLearning Landing Page - responsive using HTML and CSS

Greeting, Friends!

If any of you have a minute to take a look at my solution to this challenge, I would be most grateful. If you notice anything that you could suggest an improvement, I would be so grateful. I could also use suggestions on:

1. How to keep from getting lost in the code (see README.md in the repository). Even though I think I keep things pretty organized, I have cost myself a great deal of time editing the wrong media query because I was at the tablet section when I should have been in the mobile section. Yikes!!!
2. Suggestions on making decisions on what kind of structures I should use to contain other structures. Eg., my <main> was used as a two-column grid (to contain the intro section). Then inside the grid structure, I used flexbox. It seemed to work, but I didn't know if you could suggest a better way to do it.
3. Anything else you can see: you can be brutal - I just want to get better. :)

Thank you fellow-frontenders!

Jeff