right now, the app uses a light theme, but i want it to be dark, but i cannot find where can i change adwaita theme to dark
It helps if you mention which app. 
In Iceweasel that easy to find. It’s in settings first page, scroll down to Language and Appearance chose Dark.
But from post title, I’d say that you’re looking for settings in Gnome Web. There doesn’t seem to be an app setting for that. You may need to change/modify the gtk theme for all gtk apps.
You can try use dconf-editor to edit gnome apps settings
thanks! i’ll try that
about what app: that’s Gnome Web, Epiphany, and its just named “Wed” in haiku and uses Adwaita