Changing Gnome 3 interface default fonts

GLOBAL
Put this in your (create) /etc/fonts/local.conf file:

<?xml version='1.0'?>
<!DOCTYPE fontconfig SYSTEM 'fonts.dtd'>
<fontconfig>
<match target="pattern">
<test qual="any" name="family"><string>Cantarell</string></test>
<edit name="family" mode="assign" binding="same"><string>Open Sans</string></edit>
</match>
</fontconfig>

LOCAL
setting font size of the top bar:

1. "Activate the "User Themes" gnome-shell extension
2. Create the ~/.themes/CustomFontTheme/gnome-shell directory
3. Create gnome-shell.css containing:
   #panel {
   font-size: 14px;
   }
4. Open Gnome Tweak Tool and select your CustomFontTheme."