The sidebar is always transparent, there is no other option. Unfortunately
the sidebar seems easily corrupted.
Try these two solutions:
First: Open sidebar properties (in the control panel) and make sure the
"keep on top" box is NOT checked
-close (exit) the sidebar
-Goto sidebar folder C Driver > Programs Files > Windows Sidebar
-rename the settings.ini to settings.ini.old
**NOTE - you will need to open the gadgets window to reactivate your old
gadgets, when sidebar resests it will start up with the stock gadgets. All
the gadgets will be there just not shown until you add them back.
> How do I change the background of the side bar? It used to be transparent
> and now it's white. I want it transparent again.