Hi @cezareluke,
To achieve these types of design chages we’d recommend using Google Chrome Developer Tools. You can right-click anywhere on your webpage to select ‘Inspect Element’, the HTML and CSS properties show up right in your browser.
Below are some tutorials for Chrome Developer Tools that explains how to use that to locate CSS classes which you can then write custom CSS to change the colour of desired elements on a site.
https://developers.google.com/web/tools/chrome-devtools
https://www.udemy.com/course/devtools-2017-the-basics-of-chrome-developer-tools/
If you’d like to learn more about CSS, this is also a great tutorial: https://tutsplus.com/course/30-days-to-learn-html-and-css/
You can then add your created CSS to WordPress Admin > Appearance > Customize > Additonal CSS