Undoing a command in Terminal

Please help me someone, I'm at my wits end with this. I update to Tahoe the other day, wasn't keen on the layout cos of the new 'liquid glass' thing, so read that you can turn it off. I went into terminal and copied & pasted something and now it's changed my dock bar. It's lost it's frame. Not only that, my pop up boxes have gone funny and you can't read them (e.g when it says to shut down). I will try and enclose some screenshots, not sure if I can as I've never been on here before.