I just checked out multiple themes with Budgie Desktop Settings transparency. You can either have always, dynamic, or none. You can't set the specific transparency level you want. It is dependant upon the theme.
I deleted the gtk.css
file I described earlier, logged out, and changed my theme to Plata Noir and the transparency to Always (I switched the picture here to Dynamic before I took the screenshot on accident).
This is the result.
This is how the Plata Noir theme looks with None picked for transparency.
This image is of the Plata Noir theme with the GTK.css file added back and transparency set to 75%.
This is what the transparency looks like at 50% with the Matcha-Dark-Sea theme.
Budgie Desktop Settings allows you to have an always on or always off transparency setting. Dynamic only works in a few cases that I have seen, depending on what windows are open.. I am not very familiar with how it functions nor do I understand how it is supposed to.
However if you want to customize the Budgie Desktop and choose which color and transparency level you want for the Budgie Panel regardless of which theme you are using like the original poster wanted..
You need to change the gtk.css
located in /.config/gtk-3.0
like my initial post described.