You know, the reason why it is still in XNB is because of the ("main" / title) menus. They do not convert without you changing the original XNB files.I've converted the texture pack to the current (at least as of 1.3.5) and more user-friendly format using XNB2PNG, given how the original link disappeared and Caicyo hasn't been active for almost a year by now there shouldn't be any issue with sharing it. I suppose.
Dropbox - Flat UI.zip
Send me a warning in case there is and I'll take it down.
Follow the usual instructions to install it and don't forget that they're slightly different if you're using tModLoader.