Breeze GTK theme
The Breeze GTK Theme is what makes GTK applications look at home in KDE Plasma. To the extent that they don't, this is one of the primary places to fix those bugs.
Top bugs and feature requests
Bugs: https://bugs.kde.org/buglist.cgi?bug_severity=grave&bug_severity=major&bug_severity=crash&bug_severity=normal&bug_severity=minor&bug_status=UNCONFIRMED&bug_status=CONFIRMED&bug_status=ASSIGNED&bug_status=REOPENED&component=gtk%20theme&list_id=1629889&priority=VHI&priority=HI&product=Breeze&query_format=advanced
Feature requests: https://bugs.kde.org/buglist.cgi?bug_severity=wishlist&bug_status=UNCONFIRMED&bug_status=CONFIRMED&bug_status=ASSIGNED&bug_status=REOPENED&component=gtk%20theme&list_id=1629888&priority=VHI&priority=HI&product=Breeze&query_format=advanced