Explanation
A: You can create a custom look and feel (a theme), for your app by making changes to the default colors and visual elements provided in the uncustomized system.
C: Unmanaged Solution: The beginning state of solution is the unmanaged solution state. During this phase, you can add, edit, update, remove, delete, and test any of the components of the solution. You also have the ability to create restrictions on the components within the solution.
Note: Managed Solution: A managed solution is a finalized solution that can be distributed and installed. They are created by exporting an unmanaged solution by setting restrictions to prevent any further customizations.
The whole point of Managed is locking down the Component states so they cannot be edited.
Reference:
https://powerusers.microsoft.com/t5/Power-Apps-Pro-Dev-ISV/Managed-vs-Unmanaged/td-p/495685
https://docs.microsoft.com/en-us/powerapps/maker/model-driven-apps/create-themes-organization-branding