Update: I ended up figuring out the solution. Turns out creating a new profile in Skada and Quartz with the default settings resets the border. I'm leaving my post up in case others run into the same issue.
I tried Skinner and it looks great but in the end I decided the classic WoW look fits my style better. What's the best way to uninstall the addon?
I'm asking because even after I disabled all its skinning under options and then removed Skinner and reloaded WoW, my Skada and Quartz look different than before. The bars look rounded instead of flat in Skada and there is a border around the Skada window and the Quartz castbar when before there was none. (I can take a screenshot if needed.) I tried deleting the Skada and Quartz folders and reinstalling them, clearing my WoW cache, replacing my WTF file with an older copy from backup, and of course enabling and disabling Skinner in various ways (disabling addon skinning under options, removing the files from the SkinMe folder, disabling the addon completely, etc) and restarting WoW many times. Nothing seems to work.
I'd like to go back to my original look without having to delete and reinstall the whole game, and I'm clueless about what settings to change to accomplish this. Any help would be much appreciated. Thanks!