Bordered Card has an inconsistent background color when using the raising dark mode elevation feature

[mlaursen]

Light Theme ✅

Dark Theme ❌

This issue happens since the raising dark mode elevation has different "surface" value than the dark elevation for the current box shadow. I think to fix this I will:

• add a new $rmd-card-bordered-background-color variable that is used just for this use case (might rename)
• this variable will default to the same value as the box-shadowed cards