Changing the opacity when being used

Hi! I have been using Plank for... well... forever and I'm now comfortable enough with it to edit and create themes. Everything in the ~/.local/share/plank/themes seems straight forward enough but I can't find any info about how to change its opacity when its just sitting on the desktop vs when the mouse is inside the dock. In other words, when the dock is just sitting there, I'd like its opacity to be 90% but then when it is interacted with it becomes completely opaque at 100%. (Although if it works in reverse, the normal dock would be 10% and when interacted with it would be 0%).

Can you please explain if this is possible? If it how, how do I do it?

Thank you very much!

~Ev