[Content] Add a toggle to the header image to not overlay the site colour

[tudoramariei]

Is your feature request related to a problem? Please describe.
Some users might prefer to have no color overlayed over the image from the Header with background image block.

Describe the solution you'd like
A toggle (default on true) on the content block should allow control over whether the mix-blend-multiply overlay appears over some images, coloring them to the site's theme.

Describe alternatives you've considered
Additionally, the ability to have a color selector like the divider might benefit some users.

Additional context
Demo of what an image would look like with and without color: