How to change font, colours and more?

Hello

My theme – Coupon Theme 8.8

I need to change a few things, I opened the file stile.css and saw only this:

“NOTE THIS FILE IS NOT USED WITHIN THE THEME
THEREFORE ADDING CONTENT TO IT WILL NOT WORK”

So I have 3 questions:

1) How to change the font (in the screen shot), for example to the font “Play” from the Google
2) How to change the color of the blocks (on the screen shot)

3) If will need to change something else in the future, where can I to learn how to do it (is there any guide)?

—
Serge