I am trying to build a Kivy application with a sharp, crisp look and feel but the default Kivy UI is not exactly what I had in mind? Is there any way to create a custom theme to
A kivy developer called Andrés Rodríguez released a set of widgets based on google's material design principles. I am currently using them myself and they have surely saved me from alot of work. You can find KivyMD here (see bottom of this answer for more info). Their are some themes also based on the same principles but am yet to try them out one is FlatKivy and another is kivy material ui by Federico Curzel.
Some screenshots for kivyMD,flatkivy and kivy material ui respectively are here.
Click the highlighted links you will find a lot more info about the three projects.
The original KivyMD project by Andrés Rodríguez is no longer maintained but there is active fork of KivyMD by HeaTTheatR.