Example of how to add color variables

[lunatiqqc]

Is your feature request related to a problem? Please describe.
I want to define a color variable, such as "primary," and use it consistently in my MJML elements for attributes like color and background-color.

Describe the solution you'd like
Please provide an example demonstrating how to achieve this, either directly in MJML (if possible) or by using a templating engine.

Additional context
I understand that css-class can be used for styling, but some email clients do not support more than two classes in the class list, if I am not mistaken.

[lunatiqqc]

I found this is supported by defining attributes in the head element and using those. This was not clear to me initially.