As per your requirement, you can change from the constant file.
Open the constant file and you will find global class AppColor.
Inside that class, all colours are mentioned.
Go to lib -> Helper -> Constant -> AppColor Class
Change the colour according to your requirement and save the folder and test it.
You can change the font colour, header colour, theme main colour, banner colour from the entire application with just one change.
Drag and drop your required application logo in your assets folder.
Register your image on pubspect.yml file as mentioned in the image
Please make sure to give the proper file path format otherwise it will not work.
Change your path URL from constant->AppImages->appLogo key
Save it and hard reload the project it will reflect with your logo.