https://www.esoui.com/forums/forumdisplay.php?f=172
-> Check teh sticky thread at the top: "New to AddOn creation? - Usefull links and information"
Have a look at the ESOUI Wiki for tutorials about how to create a simple addon.
If you understand more you can have a look at the addon "FlatUI" to see how things were done.
I bet it uses the API function RedirectTexture to exchange texture files (ending on .dds) of the game with custom ones.
And then try your own
Good luck and especially much fun!