Default background rendering has to be handled on the button renderer level
because of a specific markup for Outlook (bgcolor attribute). Default text color and paddings are
handled via CSS inlining. That's why they are tested in RendererTest
I used the same background color and font color as the one defined for the button
element in WP core theme.json, and I also used similar padding values
(In core they use "calc(0.667em + 2px) calc(1.333em + 2px)")
[MAILPOET-5814]
cd274c0738