Classic Block thumbnail grid gallery displays vertically on Block themes

### Quick summary

When creating a thumbnail grid gallery using a classic block, the gallery displays vertically when using a block theme. Other types of galleries work (e.g. squared tiles).

### Steps to reproduce

1. Activate block theme
2. Add classic block to page
3. Inside the classic block, add media and select at least 2 imges
4. Create gallery and select "Thumbnail Grid" as gallery type
5. Set columns to two or more
6. Save the block and the post
7. Visit the saved post

### What you expected to happen

Gallery is expected to display horizontally with correct column count, as shown in example below with a classic theme.


### What actually happened

Activating a block theme displays gallery vertically, despite column count being set to 2.


### Impact

Some (< 50%)

### Available workarounds?

No but the platform is still usable

### Platform (Simple and/or Atomic)

Simple

### Logs or notes

_No response_

Start by reproducing the steps in a WordPress site using a block theme, then inspect the rendered classic-block thumbnail gallery and its styles alongside squared-tile galleries. Done means the Thumbnail Grid displays horizontally with the selected two-or-more column count under a block theme, without breaking other gallery types.

Written by the indexing model from the issue text.