How can my script change a Specific Font (for a specific class)?

Several things:

1. First and foremost, for simple CSS changes use Stylus. It's faster and simpler.

   In this case, the equivalent Stylus script would be:

   @namespace url(http://www.w3.org/1999/xhtml);
   
   @-moz-document domain("apps.bloomboard.com") {
       .text {
           font-family: Helvetica, sans-serif !important;
       }
   }
   

   or possibly:

   @namespace url(http://www.w3.org/1999/xhtml);
   
   @-moz-document domain("apps.bloomboard.com") {
       * {
           font-family: Helvetica, sans-serif !important;
       }
   }
   

   although setting a universal style with * should be done sparingly.

   
   

2. Don't reinvent the wheel. Most userscript engines support GM_addStyle(). Use that. Your script would become:

   // ==UserScript==
   // @name       Change annoying fonts
   // @namespace  http://use.i.E.your.homepage/
   // @version    0.1
   // @description  change annoying FaracoHandRegular font to a more readable one
   // @match      https://apps.bloomboard.com/*
   // @copyright  2012+, You
   // @grant      GM_addStyle
   // ==/UserScript==
   
   GM_addStyle ( `
       .text {
           font-family:    Helvetica, sans-serif !important;
       }
   ` );
   

   
   

3. See and read also:

   1. About CSS
   2. Understanding Style Precedence in CSS: Specificity, Inheritance, and the Cascade
   3. About CSS selectors