Fix horizontal scroll in the Theme Details section on smaller devices

[im3dabasia1]

The Theme Details section in Appearance > Themes has an x-axis overflow issue on smaller screens(Less than 650px). This is caused by an unnecessary margin on the .theme-screenshots class, leading to a horizontal scroll.

Steps to Reproduce:

1. Go to WP-Admin > Appearance > Themes.
2. Hover over any theme.
3. Click on Theme Details.
4. Observe the horizontal scroll caused by the margin-right applied to the '.theme-screenshots' class.

Trac ticket: https://core.trac.wordpress.org/ticket/62647

### Description

A margin of 30px was added to the right of the theme screenshots on larger screens (above 650px) to create a gap between the screenshots and the description. However, this margin was not being removed on smaller screens (below 650px), causing an overflow on the x-axis and resulting in a horizontal scroll.

I removed the margin for smaller screens, as it is unnecessary when the screenshots and description are stacked. This change resolves the issue.

### Steps to reproduce:

1. WP-Admin >. Appearance > Themes
2. Hover over any theme
3. Click on the 'Theme Details'
4. Notice that there is a margin-right added in the .theme-screenshots class, which is causing an overflow on the x-axis.

### Screencast:

Before the patch:

​https://github.com/user-attachments/assets/f51782f1-dac0-4fe5-a837-5e6286f76dbb

After the patch:

​https://github.com/user-attachments/assets/effadb0f-6c54-4282-857e-feeb1cef85ca

[im3dabasia1]

Video Reference: To showcase the issue, please refer to this video: ​https://drive.google.com/file/d/1wconcAWCda0uXopz-6Wl9ZMEJBeVGvSi/view

[parthvataliya]

Duplicate of #62411.