I have checked the documentation of Google Workspace Marketplace add-ons and I have not been able to find out how to change the styles of its cards and components (e.g. Header, Footer, Buttons, Paddings, Margins, Fonts, etc.). I just want to know if there's a solution to this or any document that would guide me in the right direction and if it is actually possible to do so?
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Since Google Workspace add-ons make use of cards, there's no way to style them directly as you would normally do with the editor add-ons.
However, according to the official documentation, you might benefit from trying the Google Workspace add-ons UI design kit available on Figma.