Hi Joe. This is directly from SmarterTools...

 

"SmarterTools allows customers to modify the look and feel of SmarterTools products by applying custom CSS in order to more closely match their identity. Full white-labeling the products is permitted, although any copyright notices hidden within source code comments and headers of the files must be retained. This requirement is fully defined in Section 9 of the product EULA, which can be found within the product installers and the online help for your product.For more information on how to apply custom CSS to your installation, please search in the Knowledge Base for the product specific article. For tips on determining the CSS needed to make styling changes, please see http://portal.smartertools.com/kb/a2959/use-inspect-element-to-determine-css-modifications.aspx";

Thanks for explaining further Joe. I don't believe this is available in SM 16x. Please see this post from August: https://portal.smartertools.com/community/a89458/custom-css.aspx

As Linda stated, there is no support for custom CSS. I believe they are working on allow you to custom build themes for SM. Though, this feature is right there with this fancy new API we have no documentation for :)