Edit Explorer Parts Glassy Area/Search Field Background. Note: you can also move the SM search area ans change it's size. Edit Startmenu and taskbar/startmenu/bottom part/extra settings. Scroll down and you will see setting to change for the location and size.
This will change the search field in the startmenu and also on the Explorer window. If you want to change the area around the search area on the explorer window. edit per pixel frames/window glass areas/Top glass area.
NOTE: I use a 50px wide by 24px high image for the search field image. Adjust the paint margins to line it up.