sudo snap connect mysql-workbench-community:password-manager-service :password-manager-service
Credits: here
THIS IS MORE OF A MYSQL WORKBENCH ISSUE THAN AN UBUNTU/POP!_OS ISSUE
Solution 1: Disable system-wide dark mode? (lol)
Solution 2 (the real one): Disable dark theme only for Workbench
Run Workbench with "env GTK_THEME=Adwaita" prepended; ie env GTK_THEME=Adwaita mysql-workbench %f
Change the desktop file (most probably /usr/share/applications/mysql-workbench.desktop) for a permanent change
Edited desktop file and more info here
Credits : here