Outdoors during the day only, or full time outdoors?
I wouldn't recommend that your dog is outdoors at night - here in the UK it gets far too cold for a boxer during the winter. And during the summer, it can get too hot, though that's not such an issue as long as you provide a means for your dog to cool down (a paddling pool and plenty of shade is ideal). Alas, boxers do not regulate temperature well, and are easily overcome by heat especially.
With that said, if it's a matter of keeping an eye on the house - she can actually do that from inside it

A dog as a deterrent to burglars is probably more effective inside. If they're in the back garden, a burglar would only need to enter from the front to avoid the dog... You might also consider that it is easier for someone to steal your dog from your yard than it is from your house.
Anyway, if you really want to leave her outdoors during the day, the best way to transition her to that is probably to spend a lot of time outside with her. During the weekends etc so that she gets used to being outdoors all day. You do need to be out as well, otherwise your dog is likely to protest at being out while you're in, and you'll quickly teach her to bark (not what you want). From there, you should be able to then leave her out while you're at work. No guarantees that she won't bark, but it's probably the best chance you have of aclimating her being outdoors all day.