How to change color/transparency of all desktop panels?

Tweaking Plasma

A

The KDE is using the plasma themes. You could install more themes and mix different themes.

System Settings > Workspace Appearance > Desktop Theme. Theme / Details tabs.

More can be found from the KDE Userbase: http://userbase.kde.org/Plasma/TweakingPlasma.

B

The KDE plasma objects are using the svg images. You could edit the images with a svg capable picture editor (Inkscape et. al.).

More can be found from the KDE TechBase: http://techbase.kde.org/Development/Tutorials/Plasma/Theme

--- Edit ---

More examples

http://www.kubuntuforums.net/showthread.php?56120-Customize-plasma-desktop-theme

Go to:

System Settings > Workspace Appearance > Desktop Theme

There is a Theme tab and a Details tab. On my desktop I was able to effect a nice dark background and white lettering appearance just adjusting the settings in the details. On my laptop I had to go back and forth between the Theme tab settings and the Details tab settings to get it right. Eventually after enough fiddling it worked for both.Both panels are now easy to read, and look nice.