I want to install a new theme to Plank but the folder is locked and I have no idea how to unlock it. Super new to Linux.
Are you in Files? If so, that sounds like you need to open Files by right-clicking on it and selecting "Run as Administrator," or something to that effect (I'm not on my elementary machine right now.). Then try opening the file. On a side note: if it's a theme, and it's in the proper folder, can you just change the theme by ctrl+right-clicking on plank and changing the theme in the dropdown?