Polishing an application (much like optimizing an application) shouldn't be taken on too early in its development, as it can distract getting the application working and making the user experience smooth. It is, however, a vital part of an application's development, and the careful application of colors, fonts, and animations can sometimes be the difference between success and failure.
Using color tools such as Paletton make the selection of a color-scheme much easier. It's important to also consider how color-blind people will see your application, and to ensure that the application is still usable by this sizeable portion of the population. If you know someone who has any form of color blindness, ask them to help test your choice in colors. Alternatively, use the color blindness simulations that palette design tools such as Paletton provide.
When adding animations to the application, it's a good idea to leverage the default animation systems provided by the platform. Avoid adding...