PDF Viewer enables you to choose a theme for your liking, that is used across the entire app.
Head to the apps Settings screen, or the Document Settings while viewing a document.
Chose a theme by tapping on one of the colored buttons in the "Theme" field.
You can choose between White, Light and Dark.
Note: Theming the whole app is only supported on iOS 11 with the new document browser. You still have the ability to change the appearance mode in a document, which automatically switches the UI colors when viewing documents as well on iOS 10 and 9. Please have a look at our dedicated appearance article for more information on this.