Tiled gallery: adding images button invisible on dark backgrounds

#### Steps to reproduce the issue

1. Add group block with dark background color
1. Add tiled gallery block with images
1. Note when selecting the block, the add images button isn't very visible:

On hover it's a bit better:

#### What I expected

The core gallery handles this better, we should consolidate and use the same component:

Start with the tiled gallery block and compare its add-images button with the core gallery component referenced in the issue. The work is done when the button is clearly visible on dark backgrounds and matches the core gallery's behavior.

Written by the indexing model from the issue text.