A simple CSS problem

I have not used Drupal in some years and I am finding it a little hard to work out the CSS structure
Could someone please tell me what CSS document change and what ID to use to change the fontsize of the site.name.
I am using the business responsive theme
<div id="block-business-responsive-theme-branding" class="contextual-region site-branding block block-system block-system-branding-block">

Drupal version: 

Source: https://www.drupal.org/taxonomy/term/3/feed