How to change the color and the transparence for panel and widgets?

Hi there, I like KDE and also minimalism.

I try to decluter a bit my virtual desktop and I wonder how I can change the panels and widgets colors and transparency. In this example I would like to change the hour in the bottom right corner, the opacity of the background and the color of the text.

I searched "everywhere" in the menu without seeing something useful, I tried also the manual but they are not mentionning that.

Maybe it's not possible? But if it is, can you tell me how? Thank you :)