Dark/light theme determination in Cover block omits gradient

[stokesman]

When using a gradient as the overlay color in the Cover block the theme determination does not factor in the gradient colors. Without an image, the outcome is always light-themed. With an image, if the overlay opacity is > 50 the outcome is always dark-themed. Here are example cases where this makes for poor contrast:

The useCoverIsDark hook is what determines which theme to use. I'm assuming that gradient support was added on at some point and the hook was not updated to support it. Probably a fairly challenging thing to get right. Just averaging the gradient stop colors may work well enough if they are equidistant but what to do in case they are not?

To reproduce
Steps to reproduce the behavior:

1. Go to https://wordpress.org/gutenberg/
2. Insert a Cover block
3. Insert a Columns block with empty columns into the Cover Block
4. Give the Cover block a gradient background with dark colors
5. Observe the color of the column appender buttons is dark and lacking contrast
6. Give the Cover block an image, change the gradient to light colors and ensure the opacity is > 50
7. Observe the color of the column appender buttons is light and lacking contrast

[ethanclevenger91]

The easy solution here is to have a manual toggle. This feature also doesn't seem to work very well with images that are dark to start with and don't need a heavy overlay.