Thank you David, I see that now in Henning's tip:
even more useful when you are coding LESS.
just use nested code like
#force {
...
all your custom rules can go in here
...
}
Wouldn't it be Neat if Gantry automatically added specificity to duplicate css rules in (templatename)-custom.css files? Gantry looks for the custom css but then we have to manually add specificity to make it work. It's kind of half baked.