Modify font in HTML

I’ve used Firebug and I was able to temporary modify a font color. However, I need to locate the html file that is associated with the page that I was in. How do I do that?
Pls note that I only want to modify a couple of font in a page, not the entire site or stylesheet.

Here is the site: https://mindset.dytnet.net
Here is what I want changed: the 3 headings (Free eBook, You are not alone! and My Videos) beneath the 3 circles. Each heading it’s own color.

Thank you.