Have you tried clearing you cache? There is a bug in in the IDE’s icon cache. After you restart Android Studio, the IDE will rebuild the icon cache, and the icons should appear correctly.
If this doesn’t work, you can also try resetting Android Studio to its default settings (File > Manage IDE Settings > Restore Default Settings). Beware though, as this will reset all settings and preferences – so be sure to back up any important configuration files before doing this.