Tiled Gallery block: Pick an image filter button border styling does not match surrounding buttons

### Issue

When editing a Tiled Gallery block, the 'Pick an image filter' button border has some unusual styling in comparison to its neighbours - with three sides thicker than other buttons.

Reported in internal testing: p8oabR-1Di-p2#comment-8418

Start by reproducing the Tiled Gallery block in the editor and locating the “Pick an image filter” button styles. Compare its border with the surrounding buttons; done means the three-sided thickness mismatch is removed and the button matches its neighbours.

Written by the indexing model from the issue text.