Incorrect usage of MUI is using multiple themes. It is recommended to stick to one theme for the entire app and utilize predefined MUI styles with minimal custom styling.
Customization Guidelines
Customizing a few components that do not align with the main theme is acceptable, but should be done sparingly using custom styles or inline styles instead of creating different themes.
While it's possible to define more than one theme, it is rarely necessary and generally advised against.
Material Design Compatibility
If extensive customization is required, MUI may not be the best fit as it primarily follows Google's Material Design principles. Consider alternatives if material design is not your preference.
MUI Base Option
I recently came across "MUI Base," which offers powerful components without predefined styling, potentially deviating from "material design." I am unfamiliar with this option myself.