Remove Top bar or border format from Cards and Textboxes in template

Hi dd88 
Which version of the desktop do you have ?
I use the last one and don't have this "grey row" 

However, there are some nuances at the system level:

It is indeed possible to adjust the boundaries of the white space in the visualizations using the padding settings shown in the image you attached.

+ 
Specifically, in a text box, the spacing is also a function of the font size—the larger the font, the greater the spacing. I assume this was designed to maintain some level of symmetry (just my assumption, without verification in the documentation).
This is, of course, a setting that we cannot control :

The pbix with the examples is attached.
If you have some suggestions and ideas about these issues , you can check here if somebody already posted and vote :
https://ideas.fabric.microsoft.com/ideas/search-ideas/?q=padding&currentSelected=

Or to post your own idea.