Instagram grid: Update the colors of the cover block

[carolinan]

Description
This PR updates the overlay, text, and link color of the cover block in the "Instagram grid" pattern to
make sure that the color contrast works in all style variations.

The new colors are base for the background, and contrast for the text and links.
This color combination has a high enough contrast ratio.

Closes #468

Screenshots

Testing Instructions
Insert the Instagram grid pattern
Test the pattern on all style variations.
Confirm that the text and link are readable.

Note: Tools like WAVE browser extension are not able to find the correct colors in the automated test, so it will report some errors.

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Co-authored-by: carolinan <[email protected]>

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