A GH repo (
https://github.com/acvxqs/MTS-Themes) was created for all your MTS needs. It contains:
* MTS Engines (KTE - mIRC 7.x, mtsloader - mIRC 6.x, hMTSe - mIRC 6.x)
* MTS theme files (1.0, 1.1, 1.2 and 1.3 - source: mircscripts.org)
* Old fonts used back in the day
I only know of MTS files being supported in mIRC 7.x currently by KTE 1.5 (with the regsub fix), by scripts ircN and P&P.
If anyone has working MTS engines or theme files not in the repo, let me know!
edit: Seems there's a consortium working on a new theme standard. Further reading:
http://dillingham.ws/MTS-2.0-Documentation/ (
https://github.com/jpdillingham/MTS-2.0-Documentation)