These are not the normal default dark colors. PL/SQL Developer does have a separate set of light and dark mode colors.
There are 2 methods to revert the colors to the correct (dark mode) defaults through Preferences > User Interface > Editor:
1. Click on the green "Modified" indicator at the upper right of the Syntax Highlighting color section. The indicator will go dark and when you save the preferences the default will be applied.
2. Click on a color button and select "Default" on the color selector. This will only work correctly in the latest Beta 7.