Did as per Clams suggestion, and got my "Mac Classic.tmTheme" file inside a "C:\Users\claud\AppData\Roaming\Sublime Text 3\Packages\Color Scheme - User" folder, AND selected Preferences -> Color Scheme -> Color Scheme - User -> "Mac Classic" from within ST3.
Then made this change:
and even restarted ST3. Expected to see either the selected text or background turn red, but to no avail. Am I missing something?
BTW, that was for testing only: I need to change the text and background color for the FIND highlight, to make them suit my global screen inverter.
Can anyone help? Thanks.