Featured Product and Featured Category blocks default colors don't have enough contrast

[Aljullu]

To reproduce

1. Add a Featured Product or Featured Category block into a post or page.
2. Notice text doesn't have enough contrast with the background.

Expected behavior

By default, we should make sure text and background have enough contrast.

[Aljullu]

@sunyatasattva will you be able to take a look at this one? I think there wasn't this issue before. Maybe we should change the overlay default color?

[sunyatasattva]

Sure, I'll look into it. Previously I think the default was white, so contrast worked well enough. Strange we didn't notice.

[sunyatasattva]

Result of testing, as clarification for this issue: this seems to happen only when themes override blocks CSS. In this case, Storefront shows this problem, while most other themes I tested with do not.