When you're working on a theme
When you're editing a theme, you can view it without using it on the whole desktop using metacity-theme-viewer YourThemeName
and view it on the whole desktop using gsettings set org.gnome.desktop.wm.preferences theme YourThemeName
Whenever you change the selected theme in GSettings, Metacity will load the newly-chosen theme. This is how control-center does it. But when you change a theme, as you're working on it, you might want to ask Metacity to reload the theme which is currently used on the whole desktop to reflect your changes. You can do this using the little-known metacity-message program, with the command metacity-message reload-theme. This works by sending the ClientMessage _METACITY_RELOAD_THEME_MESSAGE to the root window, in case you're interested.