How can i change the icons of applications at the dock but not change the icon of the application itself?
Changing the icons in the dock will happen if you change the icons of the application first. Dock icons are like shortcuts to access your applications. I don't know if there are tweaks on this that i don't know.
You could make aliases for your apps and then change the icons of the aliases. You'll have the small arrow on the corner though.
i guess i have to change the app icon then. how can i convert it back to its default icon once ive changed it btw?
First, open your icon using preview then copy it or press cmd c. leave it there first. then Go to Applications Folder>Select App>press cmd I to get info, on the upper left icon - select it, notice there's a blue color shade already, when it is, press cmd v or paste. this will change your icon. this will not change your icon in the dock yet but when you open that app, it will automatically change. i think there's already an existing thread on this or in relation. check it out.
To add to what ijingo said, to revert the icon back to the original, simply highlight the icon in the Get Info window again and hit 'delete'.
There's also CandyBar to make icon-changing easy (at least in terms of implementing changes system-wide vs. changing/deleting icons one-by-one). So far, the trial version is enough for me (since I've only opened it a few times after installing) and I still have a couple of days left.