As title states, I want to make a theme and then have it as the default.
So,
every time I open Architect my default theme is there,
every time I create a new menu, my menu background is there (not the default blue one).
Can this be done?
Since I've never worked with themes, I really don't have a conclusive answer for you. But as a workaround you could build a basic project with the theme you want and any assets common to your projects then save it somewhere as your "default" starting project. When you want to start another project with that theme, just launch the default project and immediately do a save-as to your current project name.
About the only way I can see you changing the default theme is by going to C/USERS/[username]/APPDATA/ROAMING/SONY/DVD ARCHITECT/[version]/THEMES and then renamed your theme as DEFAULT.THM.
I built a custom template that I use for virtually every DVD/BD project. I open that and all the defaults are there. Not sure if this is the same as a theme, but it gives me the backgrounds, music, font, etc. that I want.