Eclipse: Added jfxrt.jar to build path but errors still show up

Question

I want to run JavaFX on Windows Vista 32-bit. As my Eclipse Luna with java 1.8 is installed I added JavaFX to the project build path under

Project, Properties, Java Build Path, Add External JARs..., C:\Program Files\Java\jre1.8.0_191\lib\ext\jfxrt.jar

However, the following line is still underlined in red as an error by eclipse:

import com.sun.javafx.application.Application;

What should I do to fix it?

Note, I cannot run a newer development environment because Windows Vista 32-bit is old.