1) in the webassist/themes folder, here is a subfolder for each available theme.
in each of those subfolders, there is a file named theme_settings.xml. The available fonts are set in the following code:
<wafontface>
<option value="" >none</option>
<option value="Arial,Helvetica,sans-serif" >Arial,Helvetica,sans-serif</option>
<option value="Verdana, Geneva, sans-serif" >Verdana, Geneva, sans-serif</option>
<option value="Courier New, Courier, monospace" >Courier New, Courier, monospace</option>
</wafontface>
you could add you own font family to this list. you would need to edit each themes theme_settings.xml including the one in the temp_editing folder.
the base templates for the power store themes are in the Templates directory under the folder for the theme yo are using.