[Canvas] Update asset manager background image for dark mode

[ryankeairns]

The Canvas "transparent" background pattern used in the assets manager doesn't work so well in dark mode:

Dark mode (too much contrast with foreground image)

Light mode (lighter color; better for contrast with foreground image)

Originally posted by @cchaos in #32728 (comment)

[elasticmachine]

Pinging @elastic/kibana-design

[elasticmachine]

Pinging @elastic/kibana-canvas

[crob611]

@ryankeairns The background grid pattern is using #DDD and whatever background color it is over to form the grid, so in dark mode it is #1D1E24.

Here is forcing the grid to stay #FFF/#DDD in dark mode. Does this seem reasonable for every theme, or do you think there is some theme color combination we could use to keep it inline with the theme? It seems like we'd always end up with something darker than we would like if we were using some theme combination.

[ryankeairns]

@crob611 Thanks for looking into this. I think this is a special case where that background color should stay the same regardless of theme, as you noted.