Node-RED 5.0.6 Dark Theme is removing custom color settings from groups

Current Behavior

When switching to the dark theme (either by selecting it in settings or automatically) the custom group colors are removed. Custom group label font colors and group border colors appear to remain unchanged.

Expected Behavior

Custom group colors should remain after changing to the dark theme.

Steps To Reproduce

1. Open the Node-RED editor in a browser (this was tested in both Safari and Chrome on a Mac)
2. Go to Settings and change the appearance setting to either dark or auto
3. When the appearance changes from light to dark the custom group colors will disappear
4. Change to the light setting and the custom colors will return

Environment

• Node-RED version: 5.0.6
• Node.js version: v22.23.2
• npm version: 10.9.8
• Platform/OS: Node-RED running on Rpi 400 / Debian GNU/Linux 12 (bookworm)
  Node-RED editor running on MacBook Pro M4 Max / MacOS Golden Gate 27 Dev Beta 8
• Browser: Tested in both Safari and Chrome

Reproduce the issue in the Node-RED editor by switching between light, dark, and auto appearance settings with a group that has custom colors. Trace the editor's group-color handling during the theme change and identify where the fill color is lost. Done means custom group fill, label font, and border colors remain unchanged across theme switches, with a regression test covering the behavior.