As we know that AI2 still uses some legacy UI (And its distros too use) .Is there any way that we can intergrate material ui 3 without doing it manually
Actually App Inventor is not using material UI but it is using device default theme, which is by default material UI for stock android device on API >= 21 but in other device like samsung is using their own UI which is one UI so in Samsung device you will see one UI interface.
Also, App Inventor is mainly an educational tool and should be kept as simple as possible. Does user graphics designing benefit the educational mission? Not really.