ui: card - keep consistency with example-viewer

[catlabs]

I'm submitting a...

[ ] Regression (a behavior that used to work and stopped working in a new release)
[ ] Bug report  
[x] Feature request
[ ] Documentation issue or request
[ ] Support request => Please do not submit support request here, instead see https://github.com/NationalBankBelgium/stark/blob/master/CONTRIBUTING.md#got-a-question-or-problem

Current behavior

The header of the example-viewer card has a blue background while mat-card doesn't have any.

Expected behavior

Either all cards should have the same layout or we should define some classes. UX wise in the second case, we should agree on what type of behavior is behind each layout.