SUMMARY
Most of the time I have the information tab open in the sidebar to display various EXIF information of my photos. I use a normal (possibly the default) light plasma colour scheme, i.e. black text on light-grey background.

STEPS TO REPRODUCE
1. View a photo and display the sidebar’s information tab. 
2. Go to fullscreen.

OBSERVED RESULT
Gwenview switches to a dark theme. The headings, such as „File name“ and „File size“, do not change its colour. Only the actual information does. Only after changing to another photo are (or refreshing the current one) the headings redrawn and become readable against the background again.
The same happens in the other direction: after exiting fullscreen, the background colour in the panel switches from dark grey to light gray, making the headings almost invisible. Only after displaying another image are they redrawn.

EXPECTED RESULT
The headings must be redrawn after toggling fullscreen mode.

SOFTWARE/OS VERSIONS
Operating System: Arch Linux
KDE Plasma Version: 5.23.3
KDE Frameworks Version: 5.87.0
Qt Version: 5.15.2
Kernel Version: 5.14.16-arch1-2-surface (64-bit)
Graphics Platform: X11
Processors: 4 × Intel® Pentium® CPU 4415Y @ 1.60GHz
Memory: 7.7 GiB of RAM
Graphics Processor: Mesa Intel® HD Graphics 615

ADDITIONAL INFORMATION
I use QtCurve as widget set.