Administration: remove any usage of the #787c82 gray color

[afercia]

Splitting this out from #65807.

After [63399] there are 15 remaining usages of the color #787c82 across the codebase.

In some cases it is used for controls that contain only icons, for example the next / previous / close buttons in the media and themes browsers. Same for some icon-only controls in the Customizer.

These icons must meet a 4.5:1 minimum color contrast ratio.

In a few cases #787c82 is used for other things, e.g. a border.

Regardless, this color appears to be not justifiable given its limited use, and it can't be used for text or icon-only controls. After fixing all the relevant cases, the remaining usages would be maybe 2 or 3 so it's reasonable to remove it entirely.

Trac ticket: https://core.trac.wordpress.org/ticket/65995

The color #787c82 doesn't meet minimum color contrast ratio when used for text of icon-only controls (where the icon is equivalent to text that must meet the contrast threshold).

This PRs removes all usages of #787c82 in favor of darker gray shades.

## Use of AI Tools

None.

[afercia]

In the linked pr ​https://github.com/WordPress/wordpress-develop/pull/13320 all usages of #787c82 are removed in favor of darker grays. A few examples places where the color is now compliant:

• Media browser previous / next / close icons.
• Theme browser previous / next / close icons.
• Customizer accordions icons.
• Site health tabs focus.
• Switch view icons focus.

See screenshots.