Formatting font and headers

I am using the Big City Theme. I want to change the main font used for the content. I googled and in style.css I have tried to add my preferred font in every instance where there is the “font-family” code, but it never makes a difference. Which line should I be looking for?

Also I am putting subheadings/titles in amongst the content using Header 4 from the drop down menu. There seems to be an issue here though, as the next line of content immediately after it (formatted as ‘paragraph’) is right under it with no gap at all. If I add an extra para (enter) the gap is then too large. How do I increase the gap below h4?

Thank you!