Because the structure of the forms is different, you are not going to be able to use the same CSS for both. But it should be fairly simple to do this with CSS.
I have a couple times that I make the readonly forms remove the border and have a white background (to make them look less like an actual field), so I took that CSS for the basis to test trying to make the disabled appearance more obvious.
Here's some CSS that works on the classic designer (tested on version 11.0.2201.20436) to make the input, textarea, and select readonly fields a darker shade of gray:
.cf-formwrap input[readonly], .cf-formwrap input[backend-readonly] {background-color : #bbbbbb !important;}
.cf-formwrap textarea[readonly], .cf-formwrap textarea[backend-readonly] {background-color : #bbbbbb !important;}
.cf-formwrap select[readonly], .cf-formwrap select[backend-readonly] {background-color : #bbbbbb !important;}
Here's some CSS that works on the new designer (tested on version 11.0.2201.20436) to make the input, textarea, and select readonly readonly fields a darker shade of gray:
.form-q input[readonly], .form-q input[disabled] {background-color : #bbbbbb !important;}
.form-q textarea[readonly], .form-q textarea[disabled] {background-color : #bbbbbb !important;}
Note that I didn't test how these work on the archival/readonly version of the forms, just tested from the form preview.