There isn't a CSS class specifically around just the number. The CSS class is around the entire element, which will include the envelope. The envelope is a font icon so font color will change its color. To avoid this add to your CSS to avoid styling the font awesome class. Additionally this is all template driven. You can create a custom template by making a copy of the default template folder below and just directly change the HTML as needed.
/components/com_comprofiler/plugin/user/plug_pms_mypmspro/templates/default
Note your template folder only needs to contain the template files you intend to change. Once your template folder is created you can select it within Private Message System plugin parameters.