Created attachment 142584 [details]
Screenshot of described behavior.
The background of the "Compose message" box is white; it does not follow the system color scheme.
STEPS TO REPRODUCE
1. Use a dark color scheme (mine is Gruvbox Dark Red).
2. Open SMS Messages from KDE Connect menu.
3. Try to type a message (in "Compose message" text box).
Text is unreadable because of white text on a white background.
The background of the text box should follow the system color scheme.
Linux/KDE Plasma: Kubuntu 21.04
(available in About System)
KDE Plasma Version: 5.21.4
KDE Frameworks Version: 5.80.0
Qt Version: 5.15.2
*** This bug has been marked as a duplicate of bug 434791 ***