Template:HD/wikilayout

Are you asking about a sudden change to Wikipedia's appearance? It is because the default skin has changed from the Vector legacy (2010) skin to the new Vector (2022) skin. If you would like to change back to the old one, you can, as a registered user, click on the in the top-right corner and choose Preferences. Once there, go to Preferences → Appearance → Skins → Vector legacy (2010).
If you would like to leave feedback, you can do so at Wikipedia talk:Vector 2022.