Creating a .mss file - How do I create and save a custom style as a .mss file?

Hello!
I would want to create a .mss custom style, so that I could just load it whenever I begin working on a score, and not have to manually open the Style menu and modify what I want, such as changing the Title and Subtitle font from the default FreeSerif to Times New Roman, etc.
I found the "Load style" button, but it seems like I have to create a custom style first, which is in a .mss format. I've searched around trying to find a way of creating that, but with no success. Does anyone know how?
Thank you very much.