How is more suitable to edit the style of the template?

Hi! I have been coding for a few months so I know the basics of CSS and HTML. I downloaded a theme from this site and it’s a lot of code. How should I change the colors of the buttons and text? In the CSS stylesheet or in HTML with the “style” property?

Also, it’s a good idea to change the font-family or would that change the structure of the template too much? I saw that my index.html is linked to five different css stylesheets and 3 of them are just hundreds of lines code for @font-face {font-family: … } property.