Stats preview card using HTML and CSS flexbox, @media

What I found really difficult while building the projects was:

1. How to properly name my css classes.
2. How to change the background color of the image using css. I still managed to do it but i could not get the color to match exactly.
3. Positioning with css especially positioning text within flex childs I was really wingin most of the text positioning.

Areas I am unsure of: Pretty much all areas from the parts I found difficult above lol. Though I am also unsure on how I removed the default margin and padding of my website. Instead of using the '*' (asterisk) and setting the margin and padding to 0, i used bofy, html and set that margin and padding to 0. I hope that is not bad practice.

Questions I have:

1. How can I get better at naming my css classes according to industry standard?
2. What is the best way to change the background color of an image using css?
3. What can i improve on in my attempt?