Styles are applied inconsistently after toggling dark-mode with the editor open

[mkevins]

Describe the bug
Dark-mode styles are applied incorrectly when the mode is toggled during an editing session. The appender can appear white in dark-mode, and dark in light-mode, depending on the order that blocks are inserted and the mode is toggled.

To Reproduce

1. Create a new post
2. Add a group block
3. Toggle dark-mode
4. Add another group block

Expected behavior
The dark (or light) mode styles should be applied consistently across blocks, and should reflect the current state of the mode.

Screenshots
Dark-mode style bug in columns block:

This has been obvserved in group and gallery as well. Also, the placeholders in unselected inner blocks are white in darkmode.

Smartphone (please complete the following information):

• Device: Pixel 3a
• OS: Android X