I want to change fonts. How can I do this

open 

for Reviver

templates\t3_blank\tpls\blocks\head.php

for Flatastic

templates\t3_bs3_blank\tpls\ template_which_you_use

You can find embeding fonts. Change

<link href='//fonts.googleapis.com/css?family=Open+Sans' rel='stylesheet' type='text/css'>
<link href='//fonts.googleapis.com/css?family=Open+Sans:700' rel='stylesheet' type='text/css'>
<link href='//fonts.googleapis.com/css?family=Open+Sans:800' rel='stylesheet' type='text/css'>

with yours and make auto-replace in custom.css font-family: 'Open Sans', sans-serif;