Yes, "replacing" the textures. WoW's client has a process that looks for any texture files in the install folder before loading the ones in the data, allowing you to load your own textures. Don't worry, this is legitimate, you are not editing any data files. This only works for the UI, such as icons and other 2D elements. It used to work for sounds, which was changed back in
Legion BfA patch 8.2, and you can't replace anything in the 3D world.
For example, I am including two files I personally use which turns the edges of speech bubbles transparent while making the background fully opaque. You place these files in the following file path:
World of Warcraft\Interface\Tooltips
If your game was running, /reload to see the change. You may have to fully exit the game, but the recent change for recognizing new and changed addons on a reload may allow the game to load these new files as well.
To figure out which files you want to customize and where to place them, you can
follow this wiki article, as well as
this one, on how to extract all UI art assets from the game as well as
browsing this github repository to see each asset as a .png file. You can convert between .blp (the Blizzard proprietary image format) and .png using
BLPNG Converter hosted right on this site. Just edit the file as a .png then convert your edit to .blp. The converter saves the file output in the same folder you had the first file.