The MailChimp Form Plugin by ContactUs.com, allows implementing a default tab + form combination on your website, or making a custom implementation on every page of your website, as well as selecting a different template for your tab or Embeded Form version of the contact form.
If you want to fully customize your MailChimp Newsletter Form, e.g. customize the text, the tab color, the call out button message, and more, this needs to be done directly on ContactUs.com Admin Panel.
You can access your ContactUs.com admin panel, within the plugin. On the upper right corner of the plugin, you will see "Go to my Dashboard Analytics"; if you click on it, you'll be taken to your ContactUs.com Admin Panel.
Once you're there, go to the Forms Settings Panel by clicking on "Forms"
This section shows all the forms that you have created (any type). To customize/configure any form, you will have to select it first. Then navigate through the form's settings tabs and configure it as you need. Don't forget to save and Publish your changes.
This is how the "Form Settings" tab looks. Each tab allows customizing different sections of your contact form, such as business information, additional settings just like enabling international phone numbers, and more.
Once you have finished configuring every section, save and publish your settings. Go back to the plugin, select your implementation, e.g. tab version or Embeded Formversion, save the settings and the form will go live.
Here is a helpful video that will help you with the form configuration.