I think the priority should be lowered to GTK_STYLE_PROVIDER_PRIORITY_THEME, instead of the current GTK_STYLE_PROVIDER_PRIORITY_APPLICATION.

The problem is mostly historical.

Dark applications like gnome-photos have been using a different style class (say photos-entry-tag) and they use GTK_STYLE_PROVIDER_PRIORITY_APPLICATION load their CSS. Historically, the CSS was the same except the values of entry_tag_bg and entry_tag_fg. If the built-in light CSS uses the same priority, then it overwrites the colour definitions from the application.

Of course, the long term solution is to include the dark variant inside libgd itself, but it would be polite to let an application easily override the default.