Created attachment 149001 [details]
Appearance after the accent color is changed but before Dolphin is restarted. Folder on the left still has old accent color

SUMMARY
When using an icon theme whose folder icons respect the accent color (such as default Breeze nowadays), the color of folder icons in an open dolphin window is immediately updated when the accent color changes, but not if said folder icon has a thumbnail preview for the files inside it on it. See screenshot.

STEPS TO REPRODUCE
1. Use Breeze icons. 
2. Have one folder that contains an image and thus generates a thumbnail, and one that does not.
3. Change the accent color and be disappointed
4. Restart Dolphin

OBSERVED RESULT
The color of "normal" folder icons (like the one on the right in my screenshot) updates as expected.
The color of folder icons featuring thumbnails for the pictures inside the folder (like the one on the left in my screenshot) is not updated until Dolphin is restarted.

EXPECTED RESULT
When the accent color changes, the folders with thumbnails need to be updated too. That likely means regenerating the thumbnail for that folder on the fly.

SOFTWARE/OS VERSIONS
KDE Neon Unstable
KDE Plasma Version: 5.24.80
KDE Frameworks Version: 5.95
Qt Version: 5.15.4