It is not just about the opacity. We've figured out the solution.
Go to Tools > Comment, then select the colour swatch to 'change colour'. Select the 'gear' at the top-right to open 'Highlight Properties'. You then have the choice to select the precise colour for highlighting. (The fluorescent yellow in previous version of Acrobat Pro would be the solid yellow.)
It is not just about the opacity. We've figured out the solution.
Go to Tools > Comment, then select the colour swatch to 'change colour'. Select the 'gear' at the top-right to open 'Highlight Properties'. You then have the choice to select the precise colour for highlighting. (The fluorescent yellow in previous version of Acrobat Pro would be the solid yellow.)