Hi Milos,
You can follow this guide for how to add a new font: http://www.t3-framework.org/documentation/bs3-customization#add-fonts
Once done, you can change the LESS file to set the base font for your site:
[root]/templates/ja_brickstore/less/variables.less
Look for this declaration:
// Typography
// -------------------------
@font-family-sans-serif: "Karla", sans-serif;
@font-family-serif: Georgia, "Times New Roman", Times, serif;
@font-family-monospace: monospace;
@font-family-cursive: "Unica One", cursive;
@font-family-base: @font-family-sans-serif;
you can add new variable for new font here and set the @font-family-base as you wish.
Then you need to backup all CSS files first and go to Backend > Extensions > Templates > JA Brickstore > hit the button LESS to CSS to compile less to css.
Regards