Since I upgraded my Unity3D to Pro version its skin has changed from Light theme to Dark theme with which I am not very comfortable. Is there any way that I can switch back to Light skin?
Edit > Preferences > Colors only changes Scene colors but it has nothing the entire skin.
Try to change the skin on:
Unity Preferences → General → Editor Skin (change to Personal)
Reference: Unity - Manual: Preferences
Editor Skin | Which color scheme should Unity use for the editor? Professional users have the option of dark grey in addition to the default light grey Personal skin.