I deleted some unnecessary icon themes and now my application icons are changed. I cannot open Ubuntu software center also.... the following message is displayed....

success@user-pc:~$ software-center
2012-09-12 22:24:52,048 - softwarecenter.ui.gtk3.app - INFO - setting up proxy 'None'
2012-09-12 22:24:52,055 - softwarecenter.db.database - INFO - open() database: path=None 
use_axi=True use_agent=True
Traceback (most recent call last):
  File "/usr/bin/software-center", line 142, in <module>
    app = SoftwareCenterAppGtk3(datadir, xapian_base_path, options, args)
  File "/usr/share/software-center/softwarecenter/ui/gtk3/app.py", line 387, in __init__
    self.datadir)
  File "/usr/share/software-center/softwarecenter/ui/gtk3/panes/historypane.py", line 
78, in __init__
    self._get_emblems(self.icons)
  File "/usr/share/software-center/softwarecenter/ui/gtk3/panes/historypane.py", line 192, in _get_emblems
    pb = icons.load_icon(emblem, self.ICON_SIZE, 0)
  File "/usr/lib/python2.7/dist-packages/gi/types.py", line 43, in function
    return info.invoke(*args, **kwargs)
gi._glib.GError: Icon 'package-install' not present in theme

I also tried the following code to change the icon but no didnt work....

gksu gedit /usr/share/applications/ubuntu-software-center.desktop