pgadmin.themes.json is created in web/pgadmin/misc/themes on "yarn run bundle" and is copied to generated directory also. This file is required for themes to show up.
Currently, pgadmin.themes.json file is referred from web/pgadmin/misc/themes directory by the app. It is better to refer from generated directory instead so that it is not missed in the pgAdmin distribution phase. The pgAdmin 4.15 wheel distro is not having this file in web/pgadmin/misc/themes directory and so themes are not available in it.
Attached is the patch to set browser autofill color. I've used primary light color to highlight autofill. Note that, this works only on webkit browsers.