From the <AppDesigner doc link> way down at the bottom is the following; whether it will let you solve the problem with new version and desktop, I don't know, but implies should be able to. Why a better selection isn't the default would seeem to be worthy of an enhancement request/quality of implementation bug report.
Personalize Code View Appearance
You can customize how your code appears in the Code View editor. To change your code view settings, go to the Home tab of the MATLAB Desktop. In the Environment section, click Settings.
Change Color Settings
To change the color settings for editable sections of code and to customize syntax highlighting, select MATLAB > Appearance > Colors and adjust the desktop tool colors and the MATLAB syntax highlighting colors. These settings affect both the App Designer Code View editor and the MATLAB Editor. For more information, see Change Desktop Theme and Colors.
To change the background color of uneditable sections of code, select MATLAB > App Designer and adjust the read-only background color.
