OS X Yosemite comes with a whole new flat design and many of the existing apps are yet to be updated to match Yosemite's design and aesthetics. We can't do much to change app's design but OS X allows us to change the icons of the apps. You can use an app called LiteIcon or you can change the icons of apps manually using this guide.
I recently posted about the beautiful Glacier Icons for Yosemite and I was using those icons till yesterday. I got bored with Glacier icons and wanted to switch to default app icons. One way to do this which I knew was to drag the original icon in the app's icon in "Get Info" window of the app. Though it requires that you have the original icon backed up. The another way is to use LiteIcon app which automatically stores a copy of original icon before you change the icon of an app.
A better and much easier way to restore default icon of an app is to select the app in Applications folder, press "Cmd + I" or right-click and "Get Info" to open the Get Info window of the app. Now to restore the default icon, all you have to do is select the app's icon at the top(not the icon displayed under Preview section) and hit the delete button.
This will restore the default icon of the app. In case of restoring icons of system apps like Safari, Launchpad or System Preferences, it will ask for your password. In case of third-party apps, it may or may not ask for the authentication.