You'd want to take a look at the Panel Colorizer plasmoid.*
Edit: Oh, forgot that you want to also change the opacity of the plasmoids too - but as far as I know there is no "easy" way to do that but editing the svg files that are used to draw the plasmoids. Another reason I too think relying on SVG to draw panels and plasmoids in Plasma is a travesty and they should use QML for that.
* Yes, I still call them plasmoids because I still refuse to call them "plasma widgets"