pgadmin3 dark theme for Windows?
pgAdmin 4 v4 has an inbuilt dark theme in it.
File --> Preferences --> Miscellaneous --> Themes
Select dark theme.
You can download the latest version of pgadmin here.
Once you install, it overwrites your old version. Goodluck
The only way you can have a dark background is
File --> Options --> Query tool --> Colours
select a custom background and also change the font colour according to the background. pgadmin documentation
Update:
In pgadmin4: File --> Preferences --> Miscellaneous --> Themes