Dark mode: Placeholders in Social Icons block not visible with some block-based themes

[SiobhyB]

Describe the bug

When dark mode is enabled and a block-based theme with a light background colour is active, the placeholder icons shown for inactive icons are not visible in the Social Icons block.

To Reproduce

Steps to reproduce the behavior:

1. Enable dark mode on your iOS or Android device and open the app.
2. Activate a block-based theme with a light background colour, such as Zoologist, on a WordPress.com site.
3. Navigate to the site's post editor and observe the light background defined by the block-based theme.
4. Add the Social Icons block to the editor.
5. Notice that only the default active WordPress icon is visible, the other inactive, placeholder icons are not visible.

Expected behavior

All icons in the block should be visible, with the inactive, placeholder icons in gray and the active icons in colour.

Screenshots

Expectation	Actual

Smartphone (please complete the following information):

• Device: Pixel 6
• OS: Android 12
• Version: 19.7-rc-1

Additional context

This is related to the fact that the editor's background colour doesn't change with dark mode when a block-based theme is active, as outlined in #4468.