Hi there,
This is my first crazy idea in linux. Please don't laugh at me
If you want to make your desktop taskbar look like this:

Do this:
1. Right click at the taskbar and from menu choose 'New Panel'.
2. Right click new panel and choose 'Properties'. Window should appear.
3. General tab: name can be blank / Orientation: Top / Size: 12 pixels / Expand: yes / Autohide: no / Show hide buttons: no
4. Background tab. First download this shadow png:

then select 'Background image', point to mentioned png and hit 'Close'
Should work
I don't know if it's distro-depending or screen-resolution depending. I'm in Fedora, screen res is 1600x1200.
I hope you will be able to use it and like it.
/edit: Known issues:
1. It reduces working area - this shadow is an another panel and windows in Linux can't overlap panels.
2. It doesn't work properly on themes that put borders on the menu bars (thanks ZeroS)
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