Not sure it's the best way to do it but here's how I did it:
I added these lines at the top of my main document:
\documentclass[11pt,a4paper,sans]{moderncvMyTheme}
\moderncvstyle{classicMyTheme}
\moderncvcolor{blueMyTheme}
I copied and customized the moderncv.cls class, renamed it to moderncvMyTheme.cls, saved it inside my project, and replaced the following line:
\ProvidesClass{moderncvMyTheme}
I copied and customized one of the styles, here the moderncvstyleclassic.sty style file, renamed it to moderncvstyleclassicMyTheme.sty, saved it inside my project, and replaced the following lines:
\ProvidesPackage{moderncvstyleclassicMyTheme}
Finally I copied and customized one of the color style files, here the moderncvcolorblue.sty style file, renamed it to moderncvcolorblueMyTheme.sty, saved it inside my project, and replaced the following lines:
\ProvidesPackage{moderncvcolorblueMyTheme}
Hope that helps!